GitLab

This patch covers two changes:

- Links that refer to GitHub are updated to point to the
  relevant content on trustedfirmware.org

- Internal links between documents have been updated,
  with the required .rst suffix being added or due
  to a change in the relative locations of the files.

Change-Id: Ibf87da7d2ece726d1c94a9b33a2bbc3129de7779
Signed-off-by: Joel Hutton <Joel.Hutton@Arm.com>
Signed-off-by: Paul Beesley <paul.beesley@arm.com>