GitLab

- Add links to PULL_REQUEST_TEMPLATE.md
- Clean `System information` table

It's easier to find needes documentation about
PR process with links.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #7539