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.

[StevenMaude]

Candidates for additional features

Speeding up interaction

• Instant preview
• Instant prefetching

Layout improvements

• Footnote tooltips
• Navigation path
• Typeset plugin

Making it easier for users to work with the site

• Code selection button
• Lots of enhancements to tags, although we don't yet use tags

Site build enhancements

• Projects plugin (#1317)
• Privacy plugin
• Optimize plugin — although maybe Cloudflare Pages does something here already?

[squidfunk]

Author of Material for MkDocs here. Thanks for supporting the project financially via sponsoring! We'd be happy to learn about your experiences with the projects plugin and get your feedback to make it even better. Feel free to post on our discussion board if you need help or want to give feedback ☺️

[StevenMaude]

@squidfunk: Thank you for your work on your (very useful) project :heart: