Build the icons in GitHub workflow

[christianlupus]

The icons are currently not built at all. Thus, a local deployment cannot show these. In the official documentation, the icons are not either. As the exact deployment structure is not known, this PR just adds the functionality to build the images within the GitHub action but not publish it anywhere.

If required, I can extend this to store the generated images within the repo or do other things if required. Please tell me so if this was the case.

This is the second part of #7829 and part of #7646.