Document how to notice and fix if the handbook fails to pass checks

Created by: Joelkw

After hitting red main branches twice in one week, I felt like these small problems were popping up often enough that it's useful to document this part. (All of the "bad" examples in this doc are mine since I don't mind).

Recent examples of these types of problems from just the past month or so, to validate that this is a problem: 1, 2, 3, 4, 5, etc.

@virginiaulrich I would appreciate your eyes or clarity edits on this, because I've noticed you have great empathy and ability to phrase things for the less-github-familiar folks and I might have missed something important that's familiar to me.

Someday we might even want to monitor how often we hit failing checks – possibly a responsibility of the future Handbook PM ? cc @dadlerj