Clean up documentation of .env file

The `.env` file has grown a lot, and I think the documentation has not always kept up -- let's make sure the docs around it are clearer, ideally with better examples.

Also, it would be great to be able to fail with hard errors if certain env vars are not set.

My suggestion would be to have the env vars as comments in the `.env.sample` file and not as values meant to be replaced. Then if the user does not uncomment them purposefully and looks at them, it should just fail with an `Env Var not set` type error.

cc @vgeorge @geohacker

Start with the existing documentation around the `.env` file and the `.env.sample` file mentioned in the issue. Separate the request for clearer examples from the proposed hard errors for unset variables; done should mean the environment-variable guidance is clear and the desired missing-variable behavior is explicitly resolved.

Written by the indexing model from the issue text.