[9.18] chg: doc: Move Known Issues to BIND9 wiki

Keeping the Known Issues as part of the rendered docs has the issue that
the list can't be updated on the official docs website until the next
release. This is unpractical is a high-priority issue is discovered
shortly after a release. Keep the Known Issues in wiki and simply link
to the list from the rendered docs. The wiki article can be updated at
any time as needed.

Merge branch 'nicki/move-known-issues-to-wiki-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9718