docs: reporting-issues: make people CC the regressions list

Make people CC the recently created mailing list dedicated to Linux
kernel regressions when reporting one. Some paragraphs had to be
reshuffled and slightly rewritten during the process, as the text
otherwise would have gotten unnecessarily hard to follow.

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/ac28089d710d5d41f295221bc726555ba32f4984.1617967127.git.linux@leemhuis.info


Signed-off-by: Jonathan Corbet <corbet@lwn.net>