Better Navigation / Sidebar in Docs

[PossiblyAShrub]

This has been a frequent complaint.

The rustc-dev-guide (and other mdbook) sites have a sidebar.

Readthedocs / Sphinx also have a sidebar (eg., trio docs)

Other languages also use the sidebar pattern in their docs:

• zig
• gleam

[andychu]

Yeah, I think it's worthwhile to make a prototype of this

One thing that tripped me up is how it will work on small screens and mobile ... I think our docs actually work well on mobile, not sure how the Zig/Gleam docs look there