Consider using more features of Material for MkDocs

[StevenMaude]

This is prompted by spotting the support for metadata tags in the Material for MkDocs changelog recently.

I have reviewed all the Material for MkDocs documentation to see what features there are that we are not using.

Not all of these features may be useful for the OpenSAFELY documentation: this list is not designed to be a comprehensive "to do", but a "to evaluate".

Available features

Features that are used, but maybe not consistently

• [ ] Admonitions — there are more types than we use. We tend to use "note", "warning", "danger".
• [ ] Code annotations
• [ ] Keyboard keys — we don't always use these.

Features that are maybe not used at all

• [ ] Buttons — you can render links as buttons. For example, this might be "Apply for OpenSAFELY Access".
• [ ] Table sorting
• [ ] Diagrams
• [ ] Instant loading
• [ ] Anchor tracking
• [ ] Navigation tabs
• [ ] Navigation sections
• [ ] Navigation expansion
• [ ] Navigation indexes — to use this, you have to structure your documents on file system per section.
• [ ] Search highlighting
• [ ] Search sharing
• [ ] Tags
• [ ] Repository name
• [ ] Revision date
• [ ] MathJax — but I don't think we have much, if any, mathematical content in the documentation.

Not available yet as we're not on the Insiders version

Available soon

• [ ] Linking content tabs
• [ ] Page feedback (#644)

Available a bit later

• [ ] Social cards
• [ ] Table of contents anchor following

Available much later

• [ ] Privacy plugin — bundles all third-party assets