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.
I would be interested in clarifying this to be something like

> Most lifetimes are used in places where short names like `a`, `b` and `c` are clearest, but it's often useful to have more descriptive names.

I don't like that precise phrasing, but it gets the idea across.