Use mike for versioned docs

[eshwen]

https://github.com/jimporter/mike works with MkDocs Material to easily create versioned documentation, which could be useful if there are major API changes between versions and lots of users are still on a legacy version

Local commands required to switch from plain MkDocs to Mike, and rebuild legacy versions of the docs:

mike delete --all
git checkout v0.12.0
poetry add mike
git-changelog -p github -o CHANGELOG.md  # Regenerate now-missing changelog
mike deploy --update-aliases --push 0.12 latest
git checkout v0.13.1
poetry add mike
git-changelog -p github -o CHANGELOG.md  # Update changelog
mike deploy --update-aliases --push 0.13 latest
<push new commits to gh-pages branch>