doc: Fix broken external links ...

Using Sphinx linkcheck on the TF-A docs revealed some broken
or permanently-redirected links. These have been updated where
possible.

Change-Id: Ie1fead47972ede3331973759b50ee466264bd2ee
Signed-off-by: Paul Beesley <paul.beesley@arm.com>

docs: gxbb: Add documentation ...

Change-Id: Ie2465c1ccc482bd8eb5e5a71c580543095e4ba94
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>