Improve Pull Request guidelines

• Splits PR advice into two sections.
• Add "co-authored-by" instructions.
• Add description of draft PR and when using it is appropriate.
• Reword ZFS Test Suite checklist question.
• Link to zfs-tests.sh and zloop.sh.

Reviewed-By: Marcel Schilling <marcel.schilling@mdc-berlin.de>
Reviewed-By: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-By: Richard Laager <rlaager@wiktel.com>
Co-Authored-By: Marcel Schilling <marcel.schilling@mdc-berlin.de>
Co-Authored-By: Brian Behlendorf <behlendorf1@llnl.gov>
Co-Authored-By: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Kjeld Schouten-Lebbing <kjeld@schouten-lebbing.nl>
Closes #9753