documentation: environment variables

Our contributor guide has documentation on setting and making sure environment variables work generally but not a lot of documentation for the developer looking at a specific jsx file's `window.VAR_NAME` declaration and wondering where that came from.

We should add documentation about how to identify, manipulate, and add a new environment variable to the site, so that developers have a reference guide for working with them, especially since environment variables need to be set in multiple files in order to work which can lead to complications.

Start with the contributor guide and search the repository for `window.VAR_NAME` and existing environment-variable setup across the relevant JSX and configuration files. Done means the guide explains how to identify, manipulate, and add an environment variable, including every file that must be updated.

Written by the indexing model from the issue text.