handbook: Shorten the Git section

[trasz]

Summary

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.

[pauamma_gundo.com]

I'd also s/Comitter/Committer/ in the summary (and or git commit --amend), lest you end up with a typo in the commit message.

documentation/content/en/books/handbook/mirrors/_index.adoc

591

600

Taste; I'd make it either "-o freebsd specifies..." or "The -o freebsd origin specifies..."

625

636

I'd also mention that for docs at least, GitHub pull requests will be considered. That's what the "Edit this page" link under "Resources" does.

[pauamma_gundo.com]

Word cache collision, sorry.

documentation/content/en/books/handbook/mirrors/_index.adoc

600

Taste; I'd make it either "-o freebsd specifies..." or "The -o freebsd origin specifies..."

Typo. Second option should be "The -o freebsd option specifies..."

[trasz]

Fixes from Pau.

[debdrup]

Seems like everything's been addressed, so let's get this committed.

[pauamma_gundo.com]

LGTM too now.