More last-minute documentation updates for 0.1.0

Prompted by !369 (merged) and particularly the comment that arti-hyper was an example. I didn't intend it as an example. I have polished the docs, and made one other change here.

This branch is on top of an ad-hoc merge of https://gitlab.torproject.org/nickm/arti/-/tree/010_docs with main. You probably want to review 5610cec0..docs-more (I bet gitlab won't show this very well). I can rebase it later if necessary. (I had hoped that !369 (merged) would have merge by now, but there is some kind of CI trouble.)

Setting reviewer to @nickm since I seem to be playing tennis.