style: lint for application settings

What does this merge request do and why?

This MR adds a guidance how to add application settings. It also adds a lint checker to warn us not to use os.enviorn in general.

Reference: !796 (comment 1906028774)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.