docs: Added initial PR template with directions for doc only changes and squash merges [no ci] (#7700)

This commit adds pull_request_template.md and CONTRIBUTING.md . It focuses on explaining to contributors the need to rate PR complexity level, when to add [no ci] and how to format PR title and descriptions.

Co-authored-by: Brian <mofosyne@gmail.com>
Co-authored-by: compilade <git@compilade.net>