What? You can’t contribute to the Qubes docs because there aren’t many Qubes docs? If there’s not enough of something, that’s all the more reason to contribute to it.
All what?
Depends on what it is. See the documentation style guide for what’s appropriate. It’s not a bad thing for community documentation to exist solely on the forum if that’s where it belongs. That’s a feature, not a bug.
Generally not. See this thread: Deletion policy.