[docs] Separate some pages out of the Docs section (#1626)

This PR changes the structure of the docs site's sidebar.


![image](https://github.com/tldraw/tldraw/assets/15892272/ffe1e152-c921-43f0-9ba2-d084bda5e1e3)

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.

apps/docs/content/sections.json

@@ -1,7 +1,13 @@
 [
+	{
+		"id": "getting-started",
+		"title": "",
+		"description": "",
+		"categories": []
+	},
 	{
 		"id": "docs",
-		"title": "Docs",
+		"title": "Documentation",
 		"description": "Developer documentation for tldraw.",
 		"categories": []
 	},