doc: Move documents into subdirectories

This change creates the following directories under docs/
in order to provide a grouping for the content:

- components
- design
- getting_started
- perf
- process

In each of these directories an index.rst file is created
and this serves as an index / landing page for each of the
groups when the pages are compiled. Proper layout of the
top-level table of contents relies on this directory/index
structure.

Without this patch it is possible to build the documents
correctly with Sphinx but the output looks messy because
there is no overall hierarchy.

Change-Id: I3c9f4443ec98571a56a6edf775f2c8d74d7f429f
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/getting_started/index.rst b/docs/getting_started/index.rst
new file mode 100644
index 0000000..23608f8
--- /dev/null
+++ b/docs/getting_started/index.rst
@@ -0,0 +1,13 @@
+Getting Started
+===============
+
+.. toctree::
+   :maxdepth: 1
+   :caption: Contents
+   :numbered:
+
+   user-guide
+   image-terminology
+   porting-guide
+   psci-lib-integration-guide
+   rt-svc-writers-guide