[Documentation] Refresh the Contributing guides
Add the details of our PR process and give community members a better picture.
Depends on D5651
Next step: Will mention about our Swag Store at the end once it goes online in February.
Test Plan: Run localhost 3001
Reviewers: catherinewu, yuhan
Reviewed By: catherinewu
Differential Revision: https://dagster.phacility.com/D6025