Better documentation of environment variables

[thomasofdenmark]

Using the todo-nodejs-mongo-aks template with a azdo pipeline.

I want to add a 3rd_api_url env for use in both frontend and backend. Reading through the code to try and figure out the best way to do this, I stumble upon a lot of magic code 🎩 which I find very hard to understand.
So I ended up using KeyVault Secrets for the 3rd_api_url in the backend. And renaming all REACT_APP_ -> WEB_APP_ instances in the frontend since I am using Vuejs instead of React. And then added the 3rd_api_url in a .env file.
This in not good solutions by any means.

So a thorough documentation with examples/how-to's would be good 🙏