Move channel reg requirement to top of IRC docs

We require channels to be registered and have the openstackinfra
account added to the access list before we'll add infra-managed
bots.

Moving this documentation to the beginning of the document and
giving them their own section in case we wish to have other
requirements in the future.

Also add link on stackforge.rst back to the IRC page.

Change-Id: Ic92f3935de19c4afef23f51257a189f2860796c2
Merge "Move channel reg requirement to top of IRC docs"