Docs fixes ...

1) Fix links in "about" page
2) Put back the "contents" page with adjusted links

Change-Id: Id09140b91df5cf0a275149801d05d8cfeeda1c6e
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>

doc: Move documents into subdirectories ...

This change creates the following directories under docs/
in order to provide a grouping for the content:

- components
- design
- getting_started
- perf
- process

In each of these directories an index.rst file is created
and this serves as an index / landing page for each of the
groups when the pages are compiled. Proper layout of the
top-level table of contents relies on this directory/index
structure.

Without this patch it is possible to build the documents
correctly with Sphinx but the output looks messy because
there is no overall hierarchy.

Change-Id: I3c9f4443ec98571a56a6edf775f2c8d74d7f429f
Signed-off-by: Paul Beesley <paul.beesley@arm.com>

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>