Update the PR template

[gdlg]

This pull request updates the .github/pull_request_template.md file to improve the clarity and usefulness of pull request descriptions and checklists. The changes focus on guiding contributors to provide better summaries and references, streamlining the checklist, and removing redundant sections.

Improvements to PR description guidance:

• Added instructions encouraging contributors to provide a more comprehensive summary, including relevant facts, design choices, and testing procedures, and clarified how to reference related issues and pull requests.

Checklist and documentation updates:

• Simplified the checklist by merging unit and integration test items, and clarified requirements for documenting manual tests and changelog updates.
• Removed redundant license section and example license header, streamlining the template for easier use.

Summary

How to test

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added the description of my changes into CHANGELOG.​
• I have updated the documentation accordingly

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below).

# Copyright (C) 2025 Intel Corporation
#
# SPDX-License-Identifier: MIT