Option to customize heading for GitHub Actions workflow job summary

[cicirello]

Summary

Added option to customize heading for GitHub Actions workflow job summary. The new input is workflow-summary-heading.

Closing Issues

Closes #131

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Improvements to existing code, such as refactoring or optimizations (non-breaking)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.