Update maintainer.cc docs now that the service is live

Outgoing SMTP has been verified to work, and the service is no longer in its
initial roll-out phase, so remove the related caveats from maintainercc.rst
and email-hosting.rst.

Also add an important note advising employed maintainers to confirm with their
employer that using a third-party domain for work is permitted, and clarify in
the email-hosting comparison that maintainer.cc runs on a commercial
third-party service (forwardemail.net) funded by the Linux Foundation.

Assisted-by: claude-opus-4-8
Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>