Ownership guide

This is a work in progress, but this should get *extensive* review, so I'm putting it up early and often.

This is the start of a draft of the new 'ownership guide,' which explains ownership, borrowing, etc. I'm feeling better about this framing than last time's, but we'll see.
"never happens after step three", that is, step 3 doesn't care if step 4 happens or not, but step 4 does care that step 3 hasn't happened.