chore: improve PR template with testing and reviewer notes sections

[Nethra246]

Summary

This pull request improves the PR template to make it clearer and more structured for contributors.

Changes Made

• Added sections for testing instructions and reviewer notes.
• Simplified and reformatted checklists.
• Ensured compliance with CONTRIBUTING and CODE_OF_CONDUCT guidelines.

Benefits

• Improves communication between contributors and reviewers.
• Ensures consistent testing and documentation across PRs.

[Nethra246]

Description:
Giskard is an open-source testing platform for AI models. It helps detect and debug model biases, errors, and vulnerabilities to make AI systems safer and more reliable.

Contribution Summary:

Identified and fixed a Python environment setup issue.

Improved dependency management in the requirements.txt and setup documentation.

Verified that the project builds and runs correctly after the fix.

Outcome:

Enhanced developer onboarding by preventing build errors during installation.

Provided a working setup flow for contributors and testers.

Screenshot Attached in PR:
Included screenshot showing successful installation and test completion.