update federalist readme with federalist-docs cloud-gov setup info

[amirbey]

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User Story

A developer user should be able to find the cloud-gov setup details on the README file.

Background (Optional)

add https://github.com/18F/federalist.18f.gov/blob/main/pages/documentation/cloud-gov-setup.md to Federalist README

Acceptance Criteria

• [ ] former cloud-gov setup documentation available on README
• [ ] Change made live via deploy of federalist

After evaluating, edit this part:

Level of effort - Medium

Implementation outline (if higher than "low" effort):

• [ ] compare README and cloud-gov setup doc and see what is missing
• [ ] update REAME with missing details