Document how the design team reviews PRs

[witha-c]

Sub-issue description

We need to review and update process documentation for how Design manages and conducts design reviews for PRs in the dotgov-design-reviews Slack channel. This is separate from how we conduct design reviews for milestones or other large bundles of related work.

Acceptance criteria

• Review existing documentation in the channel canvas, onboarding doc, and search the google drive.
• Update the descriptions of: How we've automated channel notifications and information flows, how we engage and assign reviews, review checklist(s), and how we engage with GitHub to complete the review.
• Update and link references to this process in designer onboarding

Additional context

Prepare the documentation so someone new to the team has enough information to complete a design review. Assume they have beginner knowledge of the tools but intermediate-advanced design experience.