[docs] Add table of contents to Editor page (#1642)

This PR adds a table of contents to the Editor docs page.

The Editor page is quite long, with different sections. I think that
some contents at the top will make it slightly more approachable &
usable for now.

### 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

### Release Notes

- Documentation: Added a table of contents to the Editor page.

---------

Co-authored-by: Steve Ruiz <steveruizok@gmail.com>

docs/docs/editor.mdx

@@ -12,6 +12,16 @@ keywords:
   - select
 ---
 
+##### Table of Contents
+
+- [Introduction](#introduction)
+- [State](#state)
+- [Events and Tools](#events-and-tools)
+- [Inputs](#inputs)
+- [Common things to do with the editor](#common-things-to-do-with-the-editor)
+
+## Introduction
+
 The `Editor` class is the main way of controlling tldraw's editor. You can use it to manage the editor's internal state, make changes to the document, or respond to changes that have occurred.
 
 By design, the `Editor`'s surface area is [very large](/gen/editor/Editor-class). Almost everything is available through it. Need to create some shapes? Use `editor.createShapes()`. Need to delete them? Use `editor.deleteShapes()`. Need a sorted array of every shape on the current page? Use `editor.sortedShapesArray`.