[docs] using codespaces to do PR reviews without running app locally

[shkeating]

Does this documentation relate to a specific USWDS feature?
No, it's just to help people do PR reviews even if they dont have to repo installed on their machine

Is your documentation related to a problem or inconsistency? Please describe.
No, it's more to support folks helping with PR reviews that might not be eng contrinutors, or aren't on the mahcine they normally use to contribute to this repo.

Describe the documentation you'd like
Add documentation about how to run the app in a GitHub Codespace.

Additional context
Want to make it easier for non-engineers to help with the project and add more flexibility for engineers. Lower barrier to doing PR reviews.