Enhance pull request template with description and checklist sections

[mr-raj12]

The current .github/PULL_REQUEST_TEMPLATE was added in #1924 (Dec 2016) and has never been updated. It contains only 8 lines — a thank-you message, a link to the contribution guidelines, and a note asking contributors to remove the text before submitting.

The issue template (.github/ISSUE_TEMPLATE.md) was enhanced in #3783 with structured sections, but the PR template was never similarly improved.

Proposed changes:

• Add a structured ## Description section prompting contributors to describe their changes and reference related issues
• Add a ## Checklist section derived from the contribution guidelines: target branch, tests/docs, passing tests, clean commit messages
• Move instructions into HTML comments (invisible in rendered PR) so contributors don't need to manually remove template text
• Rename to .md extension for proper markdown rendering