Add improved pull request template for contributors

[ghost]

Description

This pull request adds an improved Pull Request (PR) template to standardize contributions to the Giskard repository. The new template ensures:

• Clear and structured descriptions of PRs.
• Easy linking to related issues.
• Consistent type-of-change and checklist sections for better review and QA.
• Guidelines for testing, documentation, and additional notes.

These improvements help contributors follow a standard format, reduce review friction, and make the contribution process smoother for both maintainers and newcomers.

Related Issue

#2200 – Add a Pull Request (PR) Template to standardize contributions

Type of Change

• 📚 Documentation / examples / tutorials / dependencies update
• 🔧 Bug fix (non-breaking change)
• 🥂 Improvement (enhancement of existing functionality)
• 🚀 New feature (non-breaking)
• 💥 Breaking change (changes existing functionality)
• 🔐 Security fix

Checklist

• I've read the CODE_OF_CONDUCT.md document.
• I've read the CONTRIBUTING.md guide.
• My code follows the style guide (black, isort, pre-commit hooks).
• I have added or updated relevant docstrings.
• I have updated documentation/examples if needed.
• I have self-reviewed my code.
• This PR does not duplicate an existing issue/PR.

Additional Notes

This template provides examples, testing instructions, and structured sections to make contributions more consistent and maintainable. It is meant for all future PRs in the repository.