Create getting started documentation for local testing and eval

In #47 we now have a nice way to test things locally for evaluation:
- https://github.com/aboutcode-org/federatedcode/issues/47#issuecomment-2572888419

As a follow up, we should make that part of the standard doc.

Also from a private report:

> Small note, SECRET_KEY needs to be set in the env file of purldb and federatedcode but wasnt in the instructions at https://github.com/aboutcode-org/federatedcode/issues/47 so perhaps worth adding to documentation.