Auto-expand/collapse nav items + some other improvements

[benface]

This PR squashes a few TODOs that existed in the code, notably these:

TODOs about the navigation system:

• Auto-expand the current item's ancestors on load and when the route changes
• Remember which groups/items are expanded at every level, so that closing and re-expanding an ancestor doesn't affect its children's state
• Remember if groups/items were manually expanded (with the caret) or auto-expanded (either by clicking on its title or by navigating to it or one of its children), then auto-close those that were auto-expanded when the user navigates to a different group/item

It also updates dependencies (notably Prettier, hence the large number of files changed – sorry), tweaks the Callout styles (thanks @0xa3k5), replaces ✅ emojis with GDS (phosphor) check icons (thanks @hayderkg), and colocates our custom MDX (rehype and remark) plugins in the website package, as they were already tightly coupled to it (I converted them to TS while I was at it).

[0xa3k5]

❤️