Git and related corrections and improvements

df53ae0fdd98e9452095ac2fdaf95fcfac8c9c7f (2023-04-23):

• removed portsnap
• encourages use of Git.

This round of Git-oriented corrections and changes is, essentially,
timed to precede creation of the releng/14.0 branch. Not intended to be
comprehensive.

Attention to things such as:

• HEAD (not a branch) and main (branch)
• main (not a branch) and mainline
• repositories (there are three)
• ports latest, current (not CURRENT) and quarterly
• FREEBSD-CURRENT and FREEBSD-STABLE
• master
• things that were relevant only around the time of migration to Git
• markup
• grammar.

Under
https://docs.freebsd.org/en/articles/committers-guide/#_rebasing_your_change_against_latest_freebsd_source_tree
a list of items was misrepresented as a single paragraph with asterisk
characters in its midst.

Reviewed by: carlavilla, imp
Approved by: imp
Pull Request: https://github.com/freebsd/freebsd-doc/pull/182
Differential revision: https://reviews.freebsd.org/D40026