Lifetime guide -> ownership guide
New Guide: Ownership

This replaces the previous "Lifetimes guide," since we are discussing
things from an owernship perspective now.
auto merge of #18613 : steveklabnik/rust/ownership_guide, r=huonw

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.
Some small copy edits to the guide.

Fixes #19335.