add versioning to the docs website

[brettfiedler]

It'd be good, as we build in versioning for the scenerystack npm package, to make sure the documentation aligns in version. Material for Mkdocs supports versioning with mike. https://squidfunk.github.io/mkdocs-material/setup/setting-up-versioning/

This was, if we move from, e.g., scenerystack 1.0 to 1.1 and need to change the documentation, then anyone still on 1.0 could have the relevant documentation.

Need to implement when we're ready to call the npm package complete.

Will add the dependencies to the requirements.txt and update the mkdocs.yml. Including an override that will alert viewers when they are not on the latest version.