Create documentation for GitHub Actions

[apulsipher]

Document knowledge gained building GHA:

• Security best practices, especially using full length commit SHA described here
• Artifact attestation
• List of example actions that we include or use (e.g., actions/checkout)
• Sample workflow files with general application that could be used by just about any project (e.g., epiworld-forecasts/pre-commit.yml)

[apulsipher]

Resolved in #14