handbook: Shorten the Git section

Remove stuff that's developer-only - write Git access belongs
to Committer's Handbook, normal users don't care. Also remove stuff
that's redundant, like two examples for 'git pull', and confusing,
like the ssh URL expansion/substutution explanation. Move SSH
keys lower down; there are already SSHFP records for them. Tweak
wording.

Reviewed By: debdrup, Pau Amma
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D34061