docs: reporting-issues.rst: improved process esp. for stable regressions

Provide a shorter and easier process for users that deal with
regressions in stable and longterm kernels, as those should be reported
quickly.

To realize this in the least-confusing way and without having steps
multiple times in different places, split the 'search for existing
reports' into two. That has the additinal benefit that users will search
for them quickly when going through the step by step guide and thus will
save them trouble if the find reports.
Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
CC: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/d934c15e536bceeff5c40a126930ddf803548e08.1616181657.git.linux@leemhuis.infoSigned-off-by: Jonathan Corbet <corbet@lwn.net>