library man pages

This commit adds 10 man pages to cover all libbuxton API functions. There are 22 API functions in total, but many related functions redirect to a single page.

It is very much a work-in-progress, but I wanted to see if anyone has feedback on my approach, formatting, content, etc.

Items I'm concerned about:

* I haven't looked into how buxton_client_handle_response(3) works yet, so the information there is likely incorrect.

* 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.