docs: rework docs site to have different sections (#2686)

This PR starts putting in place the high-level changes we want to make
to the docs site.
- It makes separate sections for Reference and Examples and Community.
- Gets rid of the secondary sidebar and integrates it into the main
sidebar.
- Groups the reference articles by type.
- Pulls in the examples alongside code and a live playground so people
don't have to visit examples.tldraw.com separately.

<img width="1458" alt="Screenshot 2024-01-30 at 09 43 46"
src="https://github.com/tldraw/tldraw/assets/469604/4f5aa339-3a69-4d9b-9b9f-dfdddea623e8">

Again, this is the top-level changes and there's more to be done for the
next PR(s):
  - create quick start page
  - clean up installation page
  - add accordion to Examples page prbly
  - put fun stuff in header (from footer)
  - landing page
  - something for landing page of API
  - search cmd-k and border
  - cleanup _sidebarReferenceContentLinks
  - external links _blank
  - address potential skew issue with code examples
  - have a link to other examples (next.js, etc.)

### Change Type

- [x] `documentation` — Changes to the documentation only[^2]

### Test Plan

1. Make sure examples work!

### Release Notes

- Rework our docs site to pull together the examples app and reference
section more cohesively.

---------

Co-authored-by: Taha <98838967+Taha-Hassan-Git@users.noreply.github.com>
Co-authored-by: Steve Ruiz <steveruizok@gmail.com>
Co-authored-by: Mitja Bezenšek <mitja.bezensek@gmail.com>
Co-authored-by: alex <alex@dytry.ch>
Co-authored-by: Lu Wilson <l2wilson94@gmail.com>
Co-authored-by: Dan Groshev <git@dgroshev.com>

apps/docs/utils/autoLinkDocs.ts

@@ -6,7 +6,10 @@ import sqlite3 from 'sqlite3'
 export async function autoLinkDocs(db: Database<sqlite3.Database, sqlite3.Statement>) {
 	// replace [TLEditor](?) with [TLEditor](/gen/editor/TLEditor)?
 	// not sure how we would get there but finding an article with the same title
-	const articles = await db.all('SELECT id, content FROM articles WHERE sectionId != ?', 'gen')
+	const articles = await db.all(
+		'SELECT id, content FROM articles WHERE sectionId != ?',
+		'reference'
+	)
 	await Promise.all(articles.map((a) => autoLinkDocsForArticle(db, a)))
 }
 
@@ -30,7 +33,7 @@ export async function autoLinkDocsForArticle(
 		const article = await db.get(
 			'SELECT id, sectionId, categoryId FROM articles WHERE title = ? AND sectionId = ?',
 			title,
-			'gen'
+			'reference'
 		)
 
 		if (!article) throw Error(`Could not find article for ${_title} (${title})`)