Documention on how to change environment.

Thanks for this awesome library! Have a question about changing environment variables.

I've been looking though the issues, and there are several closed issues asking how to change the environment var when building assets. As far as I can tell, there is no answer yet. Is possible for this project to document how this is accomplished?

For our project, we really need to change the environment for setting up monitoring, S3 configs and other configuration based on environment. We haven't able to do this using backpack so far. We've tried changing the NODE_ENV in `.env` (using dotEnv), changing the mode in the backpack.config, etc.

Anyways, if this is possible.. would be excellant if there could be some sort of documention on how this in done. Thanks!

------------

in .env
`NODE_ENV=production`
// run in development

```
module.exports = {
webpack: (config) => {
config.mode = 'production';
....
```
// run in development

Review the existing documentation and the webpack/backpack.config references mentioned in the issue, then trace how .env and NODE_ENV are handled during asset builds. Document the supported way to select an environment for monitoring, S3, and other configuration, with a reproducible build example as the completion check.

Written by the indexing model from the issue text.