This PR changes the structure of the docs site's sidebar.
I think this signposts more clearly what the different pages are for.
And it also paves the way for some work I want to do on
expanding+refining the Editor docs.
This PR also simplifies URL for all sidebar links.
It's a bit scrappy, but I think it feels simple enough to work with, and
easy-enough to change in the future.
> But hey! I've been doing this a couple times recently. Maybe we should
refactor? Or maybe we should keep going with what we've got and focus on
getting these docs *done*.
### Change Type
- [x] `documentation` — Changes to the documentation only[^2]
[^1]: publishes a `patch` release, for devDependencies use `internal`
[^2]: will not publish a new version
### Test Plan
1. Check that all the sidebar links go to where you expect.
2. Check that old URLs redirect to the right pages, eg: `/docs/usage`
should go to the usage page.
### Release Notes
- Documentation: Restructured the sidebar for clarity.
c5fe399842
