MkDocs build is slow

[StevenMaude]

This doesn't matter too much for an actual build during deployment, but is inconvenient for local editing.

This is primarily due to the use of the multirepo plugin. It takes over 10 seconds on my computer, which is a little bit slow when you're editing and wanting quick feedback on appearance. It used to take something like 1-2 seconds.

It apparently wants to clone the project every time you edit files; upstream issue: jdoiro3/mkdocs-multirepo-plugin#129.

Other non-ideal workarounds for local editing:

• remove the ehrQL section entirely when not working on it
• edit the repository URL to point to a local clone

#1317 might help here.