Improve reference docs for Server APIs

[bcherry]

I found this module to be quite difficult to use within Agents due to limited documentation.

This PR:

• Adds a clearer top-level explanation
• Hides some red-herring modules
• Explicitly points towards using LiveKitAPI
• Adds return types and cross links LiveKitAPI.room, egress, etc to their documentation
• Adds full docs for the RoomService client requests (I left others alone for now)
• Removes the proto imports that were misleading in the docs/source as the right way to import the proto classes is via api.* (they are re-exported in __init__.py)
• Add __aenter__ and __aexit__ to LiveKitAPI so it can be used with async with which allows cleaner example code (lmk @theomonnom if I did this wrong, I'm not super familiar)

[theomonnom]

fix CI otherwise lgtm

[davidzhao]

nice!