Cleaner API for saved/embedded visualizations + initial documentation

[brentyi]

• Fix rewind bugs
  • how to generate the web app to embed a Viser visualization in another HTML #302 (comment)
  • Playback rewind has edge cases #292
• New API (for example: deprecate set_loop_start())
• Docs

[brentyi]

Old API:

rec = server._start_scene_recording()
rec.set_loop_start()
for t in range(num_frames):
    # some updates here
    rec.insert_sleep(1 / 30)
data = rec.end_and_serialize()  # bytes

---

New API, for saving static 3D data:

data = server.get_scene_serializer().serialize() # bytes

New API, for saving dynamic data:

serializer = server.get_scene_serializer()
for t in range(num_frames):
    # some updates here
    # ...
    serializer.insert_sleep(1 / 30)
data = serializer.serialize()  # bytes