Add documentation for `sketch://add-library?url`
Add documentation for `sketch://add-library?url` here: `https://developer.sketch.com/app`

Include a TOC at the top of the page since the page is getting longer but not enough o warrant separate pages.

The Sketch code lives here: `AppController+URLHandling`

- [x] Add documentation for `add-library?url`
- [x] Add Table of Contents for page