Mkdocs tweaks

[RealOrangeOne]

mkdocs-material has a number of nice features we weren't using. Given they're there, we might as well be taking advantage of them. Specifically:

• Showing created and modified dates at the bottom of content
• Dark mode 🥳 (uses user preferences, plus with explicit toggle)
• Search suggestions
• Restore superfences for syntax highlighting, as the mermaid plugin has support for this now
• Set the font to Open Sans to match our brand
• Tweak the navbar icons, particularly repository link

Also fixes showing section indexes when you click the dropdown - this was broken since a previous update. Note that the pages were still accessible, they just required an extra click to access.

[RealOrangeOne]

(Ignore the CI failure - the diff can't be posted because it's too large).