doc: Mention the TF-A Tech Forum as a way to contact developers ...

Change-Id: Ib4ad853ebb6e28adcf9ed14714d43799f9370343
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

doc: Stop advising the creation of Phabricator issues ...

We have noticed that Phabricator (the ticketing system on tf.org [1])
has far less visibility within the community than the mailing list [2].
For this reason, let's drop usage of Phabricator for anything else than
bug reports. For the rest, advise contributors to start a discussion on
the mailing list, where they are more likely to get feedback.

[1] https://developer.trustedfirmware.org/project/board/1/
[2] https://lists.trustedfirmware.org/mailman/listinfo/tf-a

Change-Id: I7d2d3d305ad0a0f8aacc2a2f25eb5ff429853a3f
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

doc: Expand contact information in About section ...

Giving a bit more background information about the issue tracker
and mailing lists.

Change-Id: I68921d54e3113d348f1e16c685f74d32df2ca19f
Signed-off-by: Paul Beesley <paul.beesley@arm.com>

doc: Move "About" content from index.rst to a new chapter ...

The index.rst page is now the primary landing page for the TF-A
documentation. It contains quite a lot of content these days,
including:

- The project purpose and general intro
- A list of functionality
- A list of planned functionality
- A list of supported platforms
- "Getting started" links to other documents
- Contact information for raising issues

This patch creates an "About" chapter in the table
of contents and moves some content there. In order,
the above listed content:

- Stayed where it is. This is the right place for it.
- Moved to About->Features
- Moved to About->Features (in subsection)
- Stayed where it is. Moved in a later patch.
- Was expanded in-place
- Moved to About->Contact

Change-Id: I254bb87560fd09140b9e485cf15246892aa45943
Signed-off-by: Paul Beesley <paul.beesley@arm.com>