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>
15 files changed