doc: Update links within imported wiki content ...

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>

doc: Migrate a subset of the GitHub wiki content ...

With the TF wiki being migrated from GitHub to trustedfirmware.org,
some documents will be moved into the docs/ directory within the
repository rather than remaining as external content. The
appropriate action has been decided on a per-document basis.

Change-Id: Id0f615f3418369256f30d2e34e354a115389d105
Signed-off-by: Joel Hutton <Joel.Hutton@Arm.com>
Signed-off-by: Paul Beesley <paul.beesley@arm.com>