Deprecate forums, point users to replacement resources

[deoren]

Snippet from a mailing list discussion:

So how about this?

• do not mention forum any longer
• say "github is for questions"
• say "SE is experimentally for questions, but less likely to draw
  answers from team"
• keep "mailing list is great to ask questions"

update this in doc, site etc.

If nobody violently objects by monday morning, let's implement the
changes. We may not get everything immediately, but that's it. I'll
see that I can change the forum to read-only, but that needs to go to
web guys and is not top priority.

Rainer

@rgerhards @alorbach

refs rsyslog/rsyslog-doc#622