maintenance: include 'cron' details in docs

maintenance: include 'cron' details in docs

Advanced and expert users may want to know how 'git maintenance start'
schedules background maintenance in order to customize their own
schedules beyond what the maintenance.* config values allow. Start a new
set of sections in git-maintenance.txt that describe how 'cron' is used
to run these tasks.

This is particularly valuable for users who want to inspect what Git is
doing or for users who want to customize the schedule further. Having a
baseline can provide a way forward for users who have never worked with
cron schedules.

Helped-by: Eric Sunshine <redacted>
Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>