doc: update Kernel documentation build system

Update the documentation build system according to Linux v5.11-rc1.

Deactive the automarkup.py extension module which on Gitlab CI is
incompatible with Unicode.

With this patch we can build the HTML documentation using either of
Sphinx 2 and Sphinx 3.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/sphinx/maintainers_include.py b/doc/sphinx/maintainers_include.py
new file mode 100755
index 0000000..dc8fed4
--- /dev/null
+++ b/doc/sphinx/maintainers_include.py
@@ -0,0 +1,197 @@
+#!/usr/bin/env python
+# SPDX-License-Identifier: GPL-2.0
+# -*- coding: utf-8; mode: python -*-
+# pylint: disable=R0903, C0330, R0914, R0912, E0401
+
+u"""
+    maintainers-include
+    ~~~~~~~~~~~~~~~~~~~
+
+    Implementation of the ``maintainers-include`` reST-directive.
+
+    :copyright:  Copyright (C) 2019  Kees Cook <keescook@chromium.org>
+    :license:    GPL Version 2, June 1991 see linux/COPYING for details.
+
+    The ``maintainers-include`` reST-directive performs extensive parsing
+    specific to the Linux kernel's standard "MAINTAINERS" file, in an
+    effort to avoid needing to heavily mark up the original plain text.
+"""
+
+import sys
+import re
+import os.path
+
+from docutils import statemachine
+from docutils.utils.error_reporting import ErrorString
+from docutils.parsers.rst import Directive
+from docutils.parsers.rst.directives.misc import Include
+
+__version__  = '1.0'
+
+def setup(app):
+    app.add_directive("maintainers-include", MaintainersInclude)
+    return dict(
+        version = __version__,
+        parallel_read_safe = True,
+        parallel_write_safe = True
+    )
+
+class MaintainersInclude(Include):
+    u"""MaintainersInclude (``maintainers-include``) directive"""
+    required_arguments = 0
+
+    def parse_maintainers(self, path):
+        """Parse all the MAINTAINERS lines into ReST for human-readability"""
+
+        result = list()
+        result.append(".. _maintainers:")
+        result.append("")
+
+        # Poor man's state machine.
+        descriptions = False
+        maintainers = False
+        subsystems = False
+
+        # Field letter to field name mapping.
+        field_letter = None
+        fields = dict()
+
+        prev = None
+        field_prev = ""
+        field_content = ""
+
+        for line in open(path):
+            if sys.version_info.major == 2:
+                line = unicode(line, 'utf-8')
+            # Have we reached the end of the preformatted Descriptions text?
+            if descriptions and line.startswith('Maintainers'):
+                descriptions = False
+                # Ensure a blank line following the last "|"-prefixed line.
+                result.append("")
+
+            # Start subsystem processing? This is to skip processing the text
+            # between the Maintainers heading and the first subsystem name.
+            if maintainers and not subsystems:
+                if re.search('^[A-Z0-9]', line):
+                    subsystems = True
+
+            # Drop needless input whitespace.
+            line = line.rstrip()
+
+            # Linkify all non-wildcard refs to ReST files in Documentation/.
+            pat = '(Documentation/([^\s\?\*]*)\.rst)'
+            m = re.search(pat, line)
+            if m:
+                # maintainers.rst is in a subdirectory, so include "../".
+                line = re.sub(pat, ':doc:`%s <../%s>`' % (m.group(2), m.group(2)), line)
+
+            # Check state machine for output rendering behavior.
+            output = None
+            if descriptions:
+                # Escape the escapes in preformatted text.
+                output = "| %s" % (line.replace("\\", "\\\\"))
+                # Look for and record field letter to field name mappings:
+                #   R: Designated *reviewer*: FullName <address@domain>
+                m = re.search("\s(\S):\s", line)
+                if m:
+                    field_letter = m.group(1)
+                if field_letter and not field_letter in fields:
+                    m = re.search("\*([^\*]+)\*", line)
+                    if m:
+                        fields[field_letter] = m.group(1)
+            elif subsystems:
+                # Skip empty lines: subsystem parser adds them as needed.
+                if len(line) == 0:
+                    continue
+                # Subsystem fields are batched into "field_content"
+                if line[1] != ':':
+                    # Render a subsystem entry as:
+                    #   SUBSYSTEM NAME
+                    #   ~~~~~~~~~~~~~~
+
+                    # Flush pending field content.
+                    output = field_content + "\n\n"
+                    field_content = ""
+
+                    # Collapse whitespace in subsystem name.
+                    heading = re.sub("\s+", " ", line)
+                    output = output + "%s\n%s" % (heading, "~" * len(heading))
+                    field_prev = ""
+                else:
+                    # Render a subsystem field as:
+                    #   :Field: entry
+                    #           entry...
+                    field, details = line.split(':', 1)
+                    details = details.strip()
+
+                    # Mark paths (and regexes) as literal text for improved
+                    # readability and to escape any escapes.
+                    if field in ['F', 'N', 'X', 'K']:
+                        # But only if not already marked :)
+                        if not ':doc:' in details:
+                            details = '``%s``' % (details)
+
+                    # Comma separate email field continuations.
+                    if field == field_prev and field_prev in ['M', 'R', 'L']:
+                        field_content = field_content + ","
+
+                    # Do not repeat field names, so that field entries
+                    # will be collapsed together.
+                    if field != field_prev:
+                        output = field_content + "\n"
+                        field_content = ":%s:" % (fields.get(field, field))
+                    field_content = field_content + "\n\t%s" % (details)
+                    field_prev = field
+            else:
+                output = line
+
+            # Re-split on any added newlines in any above parsing.
+            if output != None:
+                for separated in output.split('\n'):
+                    result.append(separated)
+
+            # Update the state machine when we find heading separators.
+            if line.startswith('----------'):
+                if prev.startswith('Descriptions'):
+                    descriptions = True
+                if prev.startswith('Maintainers'):
+                    maintainers = True
+
+            # Retain previous line for state machine transitions.
+            prev = line
+
+        # Flush pending field contents.
+        if field_content != "":
+            for separated in field_content.split('\n'):
+                result.append(separated)
+
+        output = "\n".join(result)
+        # For debugging the pre-rendered results...
+        #print(output, file=open("/tmp/MAINTAINERS.rst", "w"))
+
+        self.state_machine.insert_input(
+          statemachine.string2lines(output), path)
+
+    def run(self):
+        """Include the MAINTAINERS file as part of this reST file."""
+        if not self.state.document.settings.file_insertion_enabled:
+            raise self.warning('"%s" directive disabled.' % self.name)
+
+        # Walk up source path directories to find Documentation/../
+        path = self.state_machine.document.attributes['source']
+        path = os.path.realpath(path)
+        tail = path
+        while tail != "Documentation" and tail != "":
+            (path, tail) = os.path.split(path)
+
+        # Append "MAINTAINERS"
+        path = os.path.join(path, "MAINTAINERS")
+
+        try:
+            self.state.document.settings.record_dependencies.add(path)
+            lines = self.parse_maintainers(path)
+        except IOError as error:
+            raise self.severe('Problems with "%s" directive path:\n%s.' %
+                      (self.name, ErrorString(error)))
+
+        return []