[Documentation] Refresh the Contributing guides

Summary:
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

Subscribers: rexledesma

Differential Revision: https://dagster.phacility.com/D6025