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/user_guides/index.rst b/docs/user_guides/index.rst
new file mode 100644
index 0000000..79bdc5d
--- /dev/null
+++ b/docs/user_guides/index.rst
@@ -0,0 +1,14 @@
+User Guides
+===========
+.. toctree::
+    :maxdepth: 1
+    :caption: Contents
+    :glob:
+    :numbered:
+
+    *
+    services/index
+
+--------------
+
+*Copyright (c) 2019, Arm Limited. All rights reserved.*
diff --git a/docs/user_guides/services/index.rst b/docs/user_guides/services/index.rst
new file mode 100644
index 0000000..94aec92
--- /dev/null
+++ b/docs/user_guides/services/index.rst
@@ -0,0 +1,12 @@
+Services
+========
+.. toctree::
+    :maxdepth: 1
+    :caption: Contents
+    :glob:
+
+    *
+
+--------------
+
+*Copyright (c) 2019, Arm Limited. All rights reserved.*