Add "Forum undergoing re-hosting" notice

[phillxnet]

We have soon to address a non trivial forum migration that will incur several possibly hours long outages and likely some periods of read-only access. With the forum down it would be reasuring for folks to have clear feedback on this process being underway/planned.

This issue is likely to come into relevant within the next few days, depending on resources. Creating here ready for the required changes to be made website wise.

[phillxnet]

This period of forum maintenance is now approaching but we do now have a sysalerts content type/section post our now completed move to Hugo (bar ongoing fixes/updates) as a result of issue #1 pr #22.

[phillxnet]

Given this is likely an ongoing issue that will repeatedly re-arise on every update, and that we now have the indicated mechanism I'll close this as fixed by #22 given it now provides the indicated mechanism. Otherwise we will be repeatedly re-opening and closing the same issue when the issue/pr maintenance is just more overhead that the mechanism is intended to reduce anyway.