doc: Convert internal links to RST format

Currently links between documents are using the format:

<path/to/><filename>.rst

This was required for services like GitHub because they render each
document in isolation - linking to another document is like linking
to any other file, just provide the full path.

However, with the new approach, the .rst files are only the raw
source for the documents. Once the documents have been rendered
the output is now in another format (HTML in our case) and so,
when linking to another document, the link must point to the
rendered version and not the .rst file.

The RST spec provides a few methods for linking between content.
The parent of this patch enabled the automatic creation of anchors
for document titles - we will use these anchors as the targets for
our links. Additional anchors can be added by hand if needed, on
section and sub-section titles, for example.

An example of this new format, for a document with the title
"Firmware Design" is :ref:`Firmware Design`.

One big advantage of this is that anchors are not dependent on
paths. We can then move documents around, even between directories,
without breaking any links between documents. Links will need to be
updated only if the title of a document changes.

Change-Id: I9e2340a61dd424cbd8fd1ecc2dc166f460d81703
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/design/interrupt-framework-design.rst b/docs/design/interrupt-framework-design.rst
index 4a864f9..d155cb3 100644
--- a/docs/design/interrupt-framework-design.rst
+++ b/docs/design/interrupt-framework-design.rst
@@ -177,10 +177,10 @@
 programmed in ``SCR_EL3`` while applying the routing model for a type of
 interrupt. The platform provides this information through the
 ``plat_interrupt_type_to_line()`` API (described in the
-`Porting Guide`_). For example, on the FVP port when the platform uses an Arm GICv2
-interrupt controller, Secure-EL1 interrupts are signaled through the FIQ signal
-while Non-secure interrupts are signaled through the IRQ signal. This applies
-when execution is in either security state.
+:ref:`Porting Guide`). For example, on the FVP port when the platform uses an
+Arm GICv2 interrupt controller, Secure-EL1 interrupts are signaled through the
+FIQ signal while Non-secure interrupts are signaled through the IRQ signal.
+This applies when execution is in either security state.
 
 Effect of mapping of several interrupt types to one signal
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -255,7 +255,7 @@
 associated interrupt numbers. It should configure the interrupt controller to
 enable the secure interrupts, ensure that their priority is always higher than
 the non-secure interrupts and target them to the primary CPU. It should also
-export the interface described in the `Porting Guide`_ to enable
+export the interface described in the :ref:`Porting Guide` to enable
 handling of interrupts.
 
 In the remainder of this document, for the sake of simplicity a Arm GICv2 system
@@ -1013,7 +1013,6 @@
 
 *Copyright (c) 2014-2019, Arm Limited and Contributors. All rights reserved.*
 
-.. _Porting Guide: ../getting_started/porting-guide.rst
 .. _SMC calling convention: http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html
 
 .. |Image 1| image:: ../resources/diagrams/sec-int-handling.png