Add Dependabot updates for GitHub Actions

[WilliamK112]

Summary

• add a minimal Dependabot version-updates configuration for GitHub Actions workflow dependencies
• schedule weekly checks for actions referenced from the repository root workflow directory

Testing

• parsed .github/dependabot.yml with PyYAML and asserted version, package-ecosystem, directory, and schedule.interval
• git diff --cached --check
• confirmed there is no existing open PR for Setup dependabot to update GHA workflow file action dependencies #54 or .github/dependabot.yml

Reference: https://docs.github.com/en/code-security/reference/supply-chain-security/dependabot-options-reference

Refs #54

[wbkboyer]

Looks good to me! 🎉