Add release notes link to changelog generation output

[mtreinish]

We should have an option to include a link to the hosted release notes in the generated changelog output. The changlog output is just a subset of the git history for the release and the full release notes contain all the details. To do this we likely need a config flag somewhere to give qiskit-bot a hint to the url for the release notes and then it can generate the version suffix for the url to link to it.

[coruscating]

I was writing up an issue for this exact feature request until I found this. Perhaps the bot can use docs_url_prefix (from Qiskit/qiskit_sphinx_theme#321) in conf.py to generate the release notes URL automatically if the format for releases in general are docs_url_prefix/release_notes.html#release-notes-version (though I know Terra has a different anchor format).