Add documentation for the brand new Collection API

4 new methods appears in the API:

- collection/getTracks
- collection/getAlbums
- collection/getArtists
- playlist/search

And 1 edited method:

- catalog/search

This PR documented them as they are implemented today. They are fresh and maybe should be marked as beta? The `source` object in getTracks and getAlbums get a notice about that.