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/design_documents/index.rst.in b/docs/design_documents/index.rst.in
new file mode 100644
index 0000000..b5cf149
--- /dev/null
+++ b/docs/design_documents/index.rst.in
@@ -0,0 +1,30 @@
+Design Documents
+================
+
+.. toctree::
+    :maxdepth: 1
+    :caption: Accepted design documents
+    :glob:
+    :numbered:
+
+    @ACCEPTED_DD_LIST@
+
+.. toctree::
+    :maxdepth: 1
+    :caption: Draft design documents
+    :glob:
+    :numbered:
+
+    @DRAFT_DD_LIST@
+
+.. toctree::
+    :maxdepth: 1
+    :caption: Rejected design documents
+    :glob:
+    :numbered:
+
+    @REJECTED_DD_LIST@
+
+--------------
+
+*Copyright (c) 2019, Arm Limited. All rights reserved.*