Doc: Added nested index structure

This patch adjusts the structure of documentation, making it
easier to organise and navigate.

* Modifies index.rst.in decoupled from cmake index.rst.
* Creates respective indexes in the documentation sub-directories
  and links them to the master index
* External component documentation is preserved
* CMAKE build system is updated to automate the document listing
  in the design_documents index instead of the master index.

Change-Id: I697b68e19dc57ecf7f36bc6dd8248fe345bdad61
Signed-off-by: Galanakis, Minos <minos.galanakis@arm.com>
diff --git a/docs/index.rst b/docs/index.rst
new file mode 100644
index 0000000..5ada824
--- /dev/null
+++ b/docs/index.rst
@@ -0,0 +1,55 @@
+:Page authors: Gyorgy Szing <gyorgy.szing@arm.com>
+
+..
+   The build-system will copy all documents into a temporary directory tree
+   before the documentation is built.
+   This file will be copied to the top level and thus please use relative paths
+   as if this file would be in <TFM_ROOT_DIR>.
+
+   The values between @ characters will be filled in by CMake.
+
+
+Trusted Firmware-M Documentation
+================================
+
+.. toctree::
+    :maxdepth: 2
+    :hidden:
+
+    Home<docs/readme>
+    docs/about/index
+    docs/design_documents/index
+    docs/user_guides/index
+    docs/processes/index
+    docs/glossary
+    docs/lic
+
+.. toctree::
+    :caption: Components
+    :maxdepth: 2
+    :glob:
+    :hidden:
+
+    lib/**
+
+.. toctree::
+    :caption: Target platforms
+    :maxdepth: 2
+    :glob:
+    :hidden:
+
+    platform/**
+
+.. toctree::
+    :caption: Tools
+    :maxdepth: 2
+    :glob:
+    :hidden:
+
+    tools/iat-verifier/*
+
+.. include:: docs/readme.rst
+
+--------------
+
+*Copyright (c) 2017-2019, Arm Limited. All rights reserved.*