docs: document git/python/Ubuntu/Debian release schedules

Going purely on upstream package release cycles doesn't tell the whole
story: a lot of people run LTS distros which will have older versions
of software we want to support.

Build out a table for us to quickly reference when making decisions as
to what versions of git/python we want to support, and when we can drop
them.  This will also help to refer users to as why we made a specific
decision that might be affecting them.

Change-Id: I7aea24bbefd50e358aeacf11e8c15a346c8fb8a9
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254572


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>