docs(acl): Add Groups and ACL documentation

[Kissaki]

Not included:

• @sub a common parent parameter description and example
• "Context" terminology as ACL rule inheritance(?) is called in the Mumble UI

Establishes terminology beyond Mumble client UI:

• [ACL] rule
• [rule] selector
• [rule] action (for allow / deny / nothing - used in text only once right now)

[Kissaki]

I had trouble confirming @sub a common parent parameter behavior and constructing/having a test case for it.

We could add that later so that we already have this documentation available.

[Kissaki]

There's opportunity for more screenshots, and reducing their default display size. But we can handle those as followup improvements.

[Kissaki]

@Krzmbrzl I resolved all your comments. Can you please take another look?

I think this would be a good starting point. We can handle additions and improvements later (add sub sibling examples, possibly reduce / adjust screenshot sizes, possibly go for more of a user -> user groups -> meta groups cascade).