Initial MiniAudio Gem documentation

Change summary

Adds initial documentation for the MiniAudio Gem and the components in it.

[x] Descriptive active voice - Do descriptive sentences have a clear subject and action verb?
[x] Answer the question at hand - Does the documentation answer a what, why, how, or where type of question?
[x] Consistency - Does the content consistently follow the Style Guide?
[x] Help the user - Does the documentation show the user something meaningful?