Add PR template for structured contributor PRs

[KH-Coder865]

What this PR does

Adds a pull request template to guide contributors toward structured and complete PR descriptions.
The template includes sections for references, code changes, user-facing changes, backwards-incompatible changes, testing, and checklist.

Why this is useful

• Improves clarity and consistency for contributor PRs
• Reduces incomplete or unstructured PR submissions
• Aligns with existing JupyterLab and Jupyter Scheduler templates
• Documentation-only change with no code impact

Related issues / references

• Inspired by PR templates in:
  • https://github.com/jupyter-server/jupyter-scheduler
  • https://github.com/jupyterlab/jupyterlab

Closes Issue

Closes: #7717

Checklist

• Code follows the project's code style
• Documentation updated (In /.github added PR template)

Labels

• enhancement

[github-actions]

👈 Launch a Binder on branch KH-Coder865/notebook/add-pr-template

[KH-Coder865]

This PR only adds a pull request template (documentation / workflow change).
No runtime or testable code is modified.
The coverage failure appears unrelated to the change itself.
I don’t have permission to add labels; once an appropriate label is added, the enforce-label check should pass.
Happy to adjust anything if needed.

[KH-Coder865]

Requesting maintainers to add the enhancement label.