docs: create a .env.example file with comments to keep track of all the required environment variables

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Is your feature request related to a problem? Please describe.

We need to create a .env.example file that documents all the required environment variables used by the app. Some environment variables, if not present, will cause the app to crash. We want to add checks for these environment variables in the app.js files that throws an error and exits with an appropriate exit code indicating the absence of a required environment variable