Similar to PolicyEngine/policyengine-app#2493. Similar to there, we should do the following:

1. Add .env and .env.example to the .gitignore
2. Add a script as part of the installation process to copy .env.example over to .env if there is not already a local .env
3. Add all relevant environment variables to .env.example with descriptions and example values. This is particularly pertinent for GOOGLE_APPLICATION_CREDENTIALS, which is needed to run the simulation API correctly