[Docs] Document events

# Feature request

#### Select the type of feature you are requesting:

- [X] Documentation

#### Describe your requested feature

Currently only RPC events are documented here https://red-discordbot.readthedocs.io/en/stable/framework_events.html .
Other custom events might be useful for cog devs.
Obviously we don't need to document the events that are in dpy already but the new ones would be nice.

List that draper made:
- ``on_message_without_command``
- ``on_red_api_tokens_update``
- ``on_filter_message_delete``
- ``on_trivia_end``
- ``on_modlog_case_edit``
- ``on_modlog_case_create``
- all the audio ones