Migrate away from Material for MkDocs

[StevenMaude]

Material for MkDocs is now in maintenance mode for 12 months commencing November 2025.

MkDocs also looks as if it's not frequently maintained.

We should consider moving to something else, which could be Zensical, the follow-up project, or something else.

[StevenMaude]

This is also tied into ehrQL and Airlock, since documentation for those projects are pulled into this main repository.