Automate removal of old `-dev0` tagged doc builds

[IAlibay]

As of writing, the MDAnalysis docs have -dev0 entries for 1.0.1, 1.0.2 (which never existed), 2.0.0, 2.1.0, 2.2.0, and 2.3.0.

Ideally the dev version should be limited to unreleased versions (i.e. 2.3.0-dev) and we should be purging records for previous dev versions whenever we do a release. I don't remember this being a problem before, have we been missing a step as part of the release process?

[lilyminium]

I never put this in as a step, so yeah it's missing. It could be done either manually as part of the release process or automated as part of docs deployment -- any preferences?

[orbeckst]

Automation is good ;-)

[IAlibay]

I've manually removed the old -dev0 builds from gh-pages and I'm adding it as a manual step for releases for now. I'll see if there's a long term action we could tag on to releases to clean this up.

edit: I'm keeping this issue open but renaming it to better reflect what needs doing.