Super minor change which allows accessing the changelog from the warning at the top from anywhere in the documentation.

Right now it's e.g. https://python-gitlab.readthedocs.io/en/stable/api/changelog.html#breaking instead of https://python-gitlab.readthedocs.io/en/stable/changelog.html#breaking when accessing it from the API reference page. This should fix that.

I simply tested it with the builtin Python server, not sure if the behavior is the same when deployed:

tox -e docs
cd build/sphinx/html
python -m http.server