Add the standard NaNLABS pull request template

[ulises-jeremias]

Summary

Add the standard NaNLABS public pull request template to this repository.

Context

A consistent PR template helps contributors describe the change, link the related issue, and provide validation evidence. This repository currently relies on convention rather than a local template.

Acceptance Criteria

• Add .github/PULL_REQUEST_TEMPLATE.md.
• Include Summary, Type of change, How was this tested?, and Checklist sections.
• Include Closes #ISSUE as the issue-reference placeholder.
• Keep the template public-safe and contributor-friendly.
• Avoid internal/private repository references.

Suggested Validation

• Preview the Markdown rendering.
• Open the template locally or in GitHub to confirm checkbox formatting works.