This ensures we have version-specific references to other projects [1].
Note that this doesn't mean the URLs are actually valid - we need to do
more work (linkcheck?) here, but it's an improvement nonetheless.

[1] https://docs.openstack.org/openstackdocstheme/latest/#external-link-helper

Change-Id: I118e4d211617c5df66ff04dc04e308a1d2fc67ad

... or "what I wish existed when I first became PTL"

Some general improvements to the contributor guide, plus new sections
for PTL duties and release management.

Change-Id: If2f3b7c18de2e6c8d9bac131a16c28c2eeb348f2