The OpenDev Manual (formerly OpenStack Infra Manual) has moved to
opendev.org, so update the link for it in the welcome message
accordingly. Do the same for the pull request closer utility, and
also reword it for better generality (a bit of de-OpenStacking).
While we're here, update outdated URLs for other content and
services which have moved to opendev.org, refresh some configuration
examples, and correct some lingering OpenStack LLC copyrights to
OpenStack Foundation per the guidance at
https://wiki.openstack.org/wiki/LegalIssuesFAQ#Copyright_Headers
(though I did not touch scripts we no longer use, like create
cgitrepos or openstackwatch).
Change-Id: Ibb34d1ab8ab58dee653c51141a871834b93cd54b
Depends-On: https://review.opendev.org/729362
bc7087425a