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/index.rst b/docs/index.rst
index a6c1ca9..f4c7386 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -1,43 +1,21 @@
-Arm Trusted Firmware-A Documentation Index
-==========================================
+Trusted Firmware-A Documentation
+================================
 
 .. toctree::
    :maxdepth: 1
-   :caption: Design Documents
+   :caption: Contents
 
-   arm-sip-service
-   auth-framework
-
-   .. toctree::
-   :maxdepth: 1
-   :caption: Release Notes
+   getting_started/index
+   process/index
+   components/index
+   design/index
+   plat/index
+   perf/index
+   security_advisories/index
+   readme
    change-log
-
-   .. toctree::
-   :maxdepth: 1
-   :caption: Guides
-
-   .. toctree::
-   :maxdepth: 1
-   :caption: Policies
-   cpu-specific-build-macros
-   exception-handling
-   firmware-design
-   firmware-update
-   interrupt-framework-design
-   platform-compatibility-policy
-   platform-interrupt-controller-API
-   porting-guide
-   psci-lib-integration-guide
-   psci-pd-tree
-   ras
-   reset-design
-   rt-svc-writers-guide
-   sdei
-   secure-partition-manager-design
-   trusted-board-boot
-   user-guide
-   xlat-tables-lib-v2-design
+   maintainers
+   acknowledgements
 
 Indices and tables
 ==================