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.

[KH-Coder865]

Requesting a review for this PR from the maintainers.

[andrii-i]

Hi @KH-Coder865. Thank you for contributing this PR.

A couple of comments:

1. Note that currently PR adds both .github/PULL_REQUEST_TEMPLATE/pull_request_template.md and .github/pull_request_template.md. We probably don't need both unless there's a good reason. I'd prefer to leave only .github/pull_request_template.md for consistency with Jupyterlab and other Jupyter projects unless there is a good reason to do it differently.
2. For the sake of consistency and reusing good practices like instructions in hidden text, could we please use this template as a base https://github.com/jupyterlab/jupyterlab/blob/main/.github/pull_request_template.md?

<!--
Thanks for contributing to JupyterLab!
Please fill out the following items to submit a pull request.
See the contributing guidelines for more information:
https://github.com/jupyterlab/jupyterlab/blob/main/CONTRIBUTING.md
-->

## References

<!-- Note issue numbers this pull request addresses (should be at least one, see contributing guidelines above). -->

<!-- Note any other pull requests that address this issue and how this pull request is different. -->

## Code changes

<!-- Describe the code changes and how they address the issue. -->

## User-facing changes

<!-- Describe any visual or user interaction changes and how they address the issue. -->

<!-- For visual changes, include before and after screenshots or GIF/mp4/other video demo here. -->

## Backwards-incompatible changes

<!-- Describe any backwards-incompatible changes to JupyterLab public APIs. -->

Thank you.