doc: Stop advising the creation of Phabricator issues

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>