Wrangle Insiders version of Material for MkDocs

[StevenMaude]

We're now sponsoring the project, and can use — or at least trial — the Insiders version.

• [x] Confirm that the license covers our use. (We believe it does as a non-profit.)
• [ ] Decide on a way of getting the code into this repository. Not sure if there's a better way other than forking via a shared bot account. Should that be into opensafely-core?
• [ ] Decide which additional features are useful for us to enable.
• [ ] Decide if we also need to get this into the ehrQL preview.