PR template: Add guidance on a good PR description

[kensimon]

We've had a lot of PR's where the description describes what the PR does, but don't give any background on why.

The style guide already outlines this:

PR descriptions should be written as if the audience has no context for the change: Explain why it's happening. Don't assume people are already aware of your feature roadmap.

So put similar guidance in the pull request template.

Type of Change

• Add - New feature or capability
• Change - Changes in existing functionality
• Fix - Bug fixes
• Remove - Removed features or deprecated functionality
• Internal - Internal changes (refactoring, tests, docs, etc.)

Breaking Changes

• This PR contains breaking changes

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• No testing required (docs, internal refactor, etc.)

Additional Notes

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 8e168918-7f43-4e7e-9d32-1997c910ae3a

📥 Commits

Reviewing files that changed from the base of the PR and between 9b2a291 and 5b44d99.

📒 Files selected for processing (1)

• .github/PULL_REQUEST_TEMPLATE.md

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Summary by CodeRabbit

• Documentation
  • Expanded pull request template guidance for describing changes.
  • Added more detailed instructions for documenting breaking changes.
  • Preserved the existing checklist and section structure.

Walkthrough

The pull request template now includes expanded guidance for writing descriptions and documenting breaking changes. Existing checklist and section structure remains unchanged.

Changes

PR Template Guidance

Layer / File(s)	Summary
Expand PR authoring instructions .github/PULL_REQUEST_TEMPLATE.md	Expanded the description placeholder instructions and breaking changes guidance, including a reminder to remove commented sections before submission.

Estimated code review effort: 1 (Trivial) | ~2 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly reflects the main change: adding guidance to the PR template for better descriptions.
Description check	✅ Passed	The description is directly aligned with the template update and explains the motivation for the guidance.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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_{Comment @coderabbitai help to get the list of available commands.}

[github-actions]

🔍 Container Scan Summary

Service	Total	Critical	High	Medium	Low	Other
boot-artifacts-aarch64	3	0	0	3	0	0
boot-artifacts-x86_64	3	0	0	3	0	0
forge-admin-cli-x86_64	255	13	30	79	7	126
machine-validation-runner	800	40	234	292	36	198
machine_validation	800	40	234	292	36	198
machine_validation-aarch64	800	40	234	292	36	198
nvmetal-carbide	800	40	234	292	36	198
TOTAL	3461	173	966	1253	151	918

Per-CVE detail lives in the per-service grype-* artifacts (JSON + SARIF). Severity counts only — no CVE IDs published here.

[kfelternv]

Should add ## Description header here so its clear where the description goes

[kfelternv]

Example where this failed: #3420