Badge request: "Used by (count)" for GitHub Actions

:clipboard: **Description**

This badge should show how many other repositories use a given GitHub Action for their workflows.

:link: **Data**

Implementation by [EndBug](https://github.com/EndBug): https://github.com/EndBug/api/blob/master/api/github-actions/used-by.ts
Example usage with also useful click action: https://github.com/EndBug/add-and-commit/blob/0c4298a18d3a9d33e3b879c7b73b4a457547d949/README.md

:microphone: **Motivation**

It would give a better measurement about usage than stars do.

Start with the referenced implementation in api/github-actions/used-by.ts and the example usage in the add-and-commit README. Check how this data could be exposed as a Shields badge and define completion as a badge showing the number of repositories using the GitHub Action, with the useful click action described in the example.

Written by the indexing model from the issue text.