Fix Broadcom Stingray platform documentation

Fix Broadcom Stingray platform documentation

 - Include the platform documentation in the table of contents.

 - Add a title for the document. Without this, the platform
   documentation was listed under a 'Description' title on page
   https://trustedfirmware-a.readthedocs.io/en/latest/plat/index.html

 - Change TF-A git repository URL to point to tf.org (rather than the
   deprecated read-only mirror on Github).

 - Fix the restructuredText syntax for the FIP command line. It was
   not displayed at all on the rendered version.

Change-Id: I7a0f062bcf8e0dfc65e8f8bdd6775c497a47e619
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>