This PR changes the paths that we use for uncategorised pages on the
docs site.
It also redirects the old paths to the new ones.
```
// BEFORE
tldraw.dev/docs/ucg/shapes
// AFTER
tldraw.dev/docs/shapes
```
I think that this is more intuitive, and will make the links that we
share a bit clearer.
eg: As a by-product, it fixes some broken links from the introduction
page.
As a trade-off, this PR makes our pathing system a bit more complicated.
But I don't think it's tooooo bad? WDYT?
### 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. Navigate to `/docs/shapes` on the docs site.
2. Make sure that it goes to the Shapes page.
3. Do the same for `/docs/ucg/shapes`
### Release Notes
- Documentation: Cleaned up some paths.
