Update developer documentation for auth updates

[acouch]

Summary

We've gotten the basics of auth working locally in https://github.com/HHS/simpler-grants-gov/tree/feature/nextjs-auth There are a couple of envars to set and a README that wasn't finished https://github.com/HHS/simpler-grants-gov/tree/feature/nextjs-auth/frontend/src/services/auth#user-auth as part of the work.

This task is to update the READMEs so for the auth work so far

Acceptance criteria

• devs should have clear steps for setting up auth work
• https://github.com/HHS/simpler-grants-gov/tree/feature/nextjs-auth/frontend/src/services/auth#user-auth is removed or cleaned up