doc: Add contents.rst to link to pages

This is the temporary contents page that links
to all other documents (except platform ports).

This page is needed during the
trustedfirmware.org migration, before we have a
Sphinx rendering pipeline set up, because cgit
doesn't offer a good way to view rendered docs
while browsing the tree. We need to have a links
page that can be opened from the cgit 'about'
view.

Change-Id: I3ad87a9fa8a14dc8e371aac7ee473575fed316bf
Signed-off-by: Paul Beesley <paul.beesley@arm.com>