doc: Start using openstackdoctheme's extlink extension

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