buxton man pages

The first commit adds two man pages, buxtonctl(1) and buxton(7), and soon four more will be added: bt-daemon(8), buxton-api(7), buxton-examples(7), and buxton-overview(7).

Depending on how lengthy buxton-overview(7) turns out to be, I may split out the discussion of layer configuration (and other topics?) into separate man pages.

Feedback is appreciated.