Don't rely on pbr ChangeLog for docs
The ChangeLog file is generated when we run `python setup.py sdist`,
which doesn't naturally happen when we run `tox -edocs`, so on a fresh
clone the docs build will fail because it references the ChangeLog file
which isn't there. Since we don't rely on pbr any more for release notes
and we have a sophisticated release note management tool, point the docs
at the published release notes instead of the pbr ChangeLog.