DevOps: Create contributing quickstart section

[cjfit]

Describe the task

The contributing section is full of great information but there's a need for a quick series of steps for CI/CD and getting code merged in. Consolidate text into a checklist so new devs can get ramped up faster:

Example:

1. Make sure you're up to date with the staging branch
2. Push your changes and open a PR with the template filled in
3. Make sure your PR passes all checks. If not, read the errors and redo.
4. Tag a code approver (see CODEOWNERS.md?) to look at your code
5. A Clean and Green Philly team member will deploy your change to the staging Vercel site.
6. If all looks good, a Clean and Green Philly team member will merge your branch into staging and close the PR. Good job! Your changes will be merged into main later manually at a later step by a team member.

Acceptance Criteria

• Clarified steps on contributing doc

Additional context