Remove dev versions automatically, or document manual removal

[lilyminium]

There should be either a way to automatically remove outdated dev versions of the user guide, or we should document how to manually do it as this should happen post stable release. FWIW the hacky way I do it is:

git checkout gh-pages

Make sure it's up-to-date:

git pull origin gh-pages

Delete the old dev version folder, and update versions.json to remove that entry. Then push back to github and check the rendered docs.