Docs feedback: Consider adding explanation of using environment variables in GitLab Runner Helm Chart docs

The documentation on GitLab Runner Helm Chart is missing information and instructions on how to set environment variables within runners using the Helm chart.

There is discussion around this feature in this merge request, which was merged about 3 years ago. I worked with a customer (internal ZD link) who had questions around implementation of this feature, and who mentioned that the docs were unclear on it.

As such, I propose that our documentation be updated to include information on this feature.