[Docs] Update docs on how to update dev and prod Firebase keys

[gino-m]

We need to update the Developer's Guide in the Wiki and any README files to indicate the keys are now stored in web/src/environments/environment.dev.ts and web/src/environments/environment.prod.ts. Dev keys are provided in this repo for use by the core team, but should be replaced with your own dev keys in forks. Placeholders are provided for prod keys, and should be replaced in your fork as well.

@amegantz Any chance you could update these?

@rfontanarosa FYI

[rfontanarosa]

done: https://github.com/google/ground-platform/wiki/Ground-Developer's-Guide