dracut: README: rewrite
The documentation in the dracut README has grown stale and inaccurate.
Remove the stale content and write a short and useful reference manual.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13012
Closes #13017