Add CustomEvent docs entry

Description

Adds docstring for CustomEvent.

I wasn't sure where to add this (I'm new to the codebase and not fluent in C++). It looks like all other doc entries correspond 1:1 with a method, however I couldn't find any method that uses CustomEvent except for RequestHandler::BroadcastCustomEvent which already has a docstring.

So, I just put another docstring above that and it seems to generate the docs correctly.

Motivation and Context

Resolves #1031

How Has This Been Tested?

Tested OS(s): MacOS 12.6

I ran build_docs.sh to visually test the changed docs

Types of changes

Documentation change (a change to documentation pages)

Checklist:

[x] I have read the Contributing Guidelines.
[x] All commit messages are properly formatted and commits squashed where appropriate.
[x] My code is not on master or a release/* branch.
[x] The code has been tested.
[x] I have included updates to all appropriate documentation.

obsproject / obs-websocket