handbook: update mirrors to include git

Pull most of URLs.md from my freebsd-git-docs github repo and reformat to
asciidoc. Copy some of the existing svn info and retarget it to Git.

Submissions on github from: lwshu@, 0mp@, Alexander Richardson, Ryan Libby,
Ceri Davies, Joseph Mingrone
Feedback from: Pau Amma, emaste@
Sponsored by: Netflix, Inc
Differential Revision: https://reviews.freebsd.org/D29252