[contributor-docs] Add guide to creating a GitHub action or workflow

This document should cover best practices that are important to follow in GitHub Actions, such as:
- Providing useful output
- Using secrets for private information (although this could be covered in “Security best practices”)
- Testing GitHub Actions
- Using Shell script vs. JavaScript

Start by locating the contributor documentation structure and any existing guidance on security best practices. Add a guide covering useful output, secrets, testing GitHub Actions, and choosing shell scripts versus JavaScript; done means each requested topic has practical best practices and the guide fits the repository’s documentation style.

Written by the indexing model from the issue text.