This PR changes how the sidebar works on the docs site.
## What's the problem?
Previously, you couldn't reach the bottom of the sidebar on some screens
- unless you scroll to the very bottom of the main content. Some of our
pages are [very
long](https://canary.tldraw.dev/gen/editor/Editor-class), so this isn't
practical.
## How are we fixing it?
This PR makes the sidebar scroll separately from the main content.
It's less fancy and less pretty :/
But it's usable.
Maybe we could make it sleeker somehow, but this works at least.
## Admin
### 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. On the docs site, make your browser window short so that you can't
see the whole sidebar.
2. Make sure that you can scroll down to the bottom on the sidebar,
separately from the rest of the page.
### Release Notes
- Documentation: Fixed the sidebar being unscrollable on some short
screens.
0c046a561b
