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.

[lucyb]

This is likely to form a larger piece of work around improving the docs, and in any case, is likely to be too large for a deck scrubbing task so I'm removing it from our board for now.

[StevenMaude]

For reference, see the Zensical roadmap and backlog.

For us to migrate this repository (along with ehrQL and Airlock) probably at least requires:

• Support mkdocs-multirepo plugin functionality zensical/backlog#26
• Support mkdocs-macros plugin functionality zensical/backlog#16
• Proposal: Subprojects zensical/backlog#44 (maybe this would suffice instead of mkdocs-multirepo, but maybe not?)

[mikerkelly]

This is likely to form a larger piece of work around improving the docs, and in any case, is likely to be too large for a deck scrubbing task so I'm removing it from our board for now.

Katie pointed out that it is now time to think about when we schedule this work, as the due date is only about 3 months away. Steve created some more detailed issues which may be the ones that we end up doing. I added the due date and put this in REX triage for discussion in planning.