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: Fixup filename and links to compatibility policy ...

Change-Id: I9d14faa7294578443233b84d5459fa7b62a30c07
Signed-off-by: Soby Mathew <soby.mathew@arm.com>