library man pages

This commit adds 22 man pages to cover all libbuxton API functions.

It is very much a work-in-progress, but I wanted to see if anyone has feedback on my approach, formatting, content, etc.

A couple of items I'm concerned about:

* I'm not sure how best to explain BuxtonKeys in buxton_key_create(3), clearly differentiating between BuxtonKeys, groups, names, group+keys.

* I don't yet have a code example for buxton_unregister_notification(3), but I think William said that he would add a demo program that I can reuse.