Enable docs versioning with mike

[ajstewart]

Enable versioning of the docs with mike, as detailed here: https://squidfunk.github.io/mkdocs-material/setup/setting-up-versioning/, which can also include a dev version of the docs.

Has been done with the pipeline, however:

• mkdocs-material>=8.2.4 must be used.
• Later version of mkdocstrings with the Griffe python handler should be used.
  • Note that some return docstrings may need fixing as it handles multiple returns slightly better than before (e.g. Tuples etc).
• This version of mkdocstrings cannot yet 'hide' the hidden functions. So either:
  • Wait for that to be enabled.
  • Fill in docstrings for hidden functions.
  • Do anyway and accept that some of the reference won't be complete.

Edit: The python-legacy handler can be used with mkdocstrings so the behaviour doesn't change.