Add sync endpoint actions for episodes

[Sporiff]

Given the new sync endpoint behavior laid out in #171, we need to define and document the actions and entities required for episode interaction. The current list is:

• Create a new episode
• Mark an episode as "played" or "unplayed"
• Update the "last played" position of an episode
• Favoriting an episode

[Sporiff]

@keunes I've added some actions here. Since you worked on this last, could you check and let me know if there are any other things we want to account for in the initial docs?