[FEAT] Add versioning to the docsite

[RossKen]

Is your proposal related to a problem?

mkdocs allows docs versioning as per https://squidfunk.github.io/mkdocs-material/setup/setting-up-versioning/

This should be helpful for tracking changes over time. Also, when major changes to the docs are made (with inevitable broken links, this should help users in the transition period)

Describe the solution you'd like

Implement mkdocs versioning as per the link above

Describe alternatives you've considered

Additional context

[NickCrews]

Note that if you want to keep it simple, Ibis just chose to do the exact opposite transition 😆

[RobinL]

I think we've decided it's more trouble than it's worth.

Legacy Splink 3 docs are
https://moj-analytical-services.github.io/splink3_legacy_docs/index.html