Use mkdocs `mike` for multi-version docs

[meatballhat]

Checklist

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What problem does this solve?

This would remove the need for multiple major version documentation trees within each major maintenance branch.

Solution description

Rework the docs to use the mkdocs mike tool as described here.

Describe alternatives you've considered

Hosting documentation at readthedocs would include support for something similar. We could not switch to mike and continue the in-tree multiple dirs approach.