Add Playlist API Developer Docs

[anultravioletaurora]

Heyo!

In working on Jellify, we're going to be performing a lot of operations around playlists, including management and collaboration

We can contribute developer documentation that outlines the Playlists API. The use cases we'd cover would be:

• Reading the playlist library, and the list of playlists
• Reading the items within a playlist
• Creating a new playlist
• Updating a playlist, such as renaming or mutating the list of items
• Deleting the playlist
• Private vs Public Playlists
• Adding playlist users