docs: fix mailing lists URLs

docs: fix mailing lists URLs

With the transition to mailman3, the URLs of TF-A and TF-A Tests
mailing lists have changed. However, we still refer to the old
location, which are now dead links.

Update all relevant links throughout the documentation.

There is one link referring to a specific thread on the TF-A mailing
list in the SPM documentation, for which I had to make a guess as to
what's the equivalent mailman3 URL. The old URL scheme indicates that
the thread dates from February 2020 but beyond that, I could not make
sense of the thread id within the old URL so I picked the most likely
match amongst the 3 emails posted on the subject in this time period.

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Reported-by: Kuohong Wang <kuohong.wang@mediatek.com>
Change-Id: I83f4843afd1dd46f885df225931d8458152dbb58