docs: link to sitemap from robots.txt (tldraw#7115)

### Change type

- [ ] `bugfix`
- [ ] `improvement`
- [ ] `feature`
- [ ] `api`
- [x] `other`

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Add `Sitemap: https://www.tldraw.dev/sitemap.xml` to
`apps/docs/app/robots.txt`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
9a4e6f8. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: mimecuvalo

apps/docs/app/robots.txt

@@ -1,2 +1,4 @@
 User-agent: *
-Allow: /
+Allow: /
+
+Sitemap: https://www.tldraw.dev/sitemap.xml