Automate mkdocs deployment

[ubdbra001]

This PR updates the existing workflow for auto-deploying the Topostats docs. It:

• Updates the docs dependencies to use mkdocs and the required plugins
• Removes the old sphinx workflow
• Adds a new workflow using mkdocs and mike to produce versioned docs

The new workflow does the following:

• On a merged PR or a manual dispatch the dev version of the docs will be updated. Older snapshots (e.g. 2.4.0) will be left as they are.
• When a new release is created (not a pre-release) the workflow will take a snapshot of the docs, add it to the list of versions available on the site using the version ID used in the release, and move the '(latest)` suffix to the most recent release of the docs.

I have tested this on my own branch and it does appear to be working correctly. I think that does everything discussed previously, but let me know if there is anything else required.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.98%. Comparing base (4e88c86) to head (82ebe8f).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1340   +/-   ##
=======================================
  Coverage   87.98%   87.98%           
=======================================
  Files          31       31           
  Lines        6026     6027    +1     
=======================================
+ Hits         5302     5303    +1     
  Misses        724      724           

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[SylviaWhittle]

🤞 that this all works - it will be great. :)