Add dracut.zfs.7

Thorough documentation with a dracut.bootup(7)-style flowchart,
dracut.cmdline(7)-style cmdline listing,
and per-file docs like the old README

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13291