add dependabot.yml

[bsadashi]

This PR adds a dependabot.yml configuration file to enable and optimize automated dependency updates for GitHub Actions workflows in this repository.


Key Features of the Configuration:

• Ecosystem: Enables Dependabot for github-actions, ensuring workflow dependencies are kept up to date.
• Directory: Applies to the repository root (/), covering all GitHub Actions workflows.
• Update Schedule: Checks for new versions and security updates on a weekly basis, balancing update frequency and review workload.
• Grouping:
  • All version updates are grouped into a single pull request, reducing noise and making it easier to review and merge changes.
  • All security updates are also grouped, ensuring prompt attention to vulnerabilities without overwhelming maintainers with multiple PRs