docs: Move pull request template documentation to its own guidance file

[c-warren]

What changed?
Moves pull request guidance from the template to its own documentation.

Why?
The comments within a git description are taking up a significant amount of space in the git log and reducing its readability. This change moves the instructions to a guidance document so that maintainers/contributors can reference the guidance without cluttering the log. It also updates our .gitar rule so that gitar will reference the new file when recommending updates to a pull request description.

How did you test it?
This PR will act as the test - gitar should pick up on the changes and correctly identify that Potential Risks was left empty.

Potential risks

Release notes
N/A

Documentation Changes
N/A

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Reviewer Validation

PR Description Quality (check these before reviewing code):

• "What changed" provides a clear 1-2 line summary
  • Project Issue is linked
• "Why" explains the full motivation with sufficient context
• Testing is documented:
  • Unit test commands are included (with exact go test invocation)
  • Integration test setup/commands included (if integration tests were run)
  • Canary testing details included (if canary was mentioned)
• Potential risks section is thoughtfully filled out (or legitimately N/A)
• Release notes included if this completes a user-facing feature
• Documentation needs are addressed (or noted if uncertain)

[gitar-bot]

Code Review ✅ Approved

Clean documentation reorganization that moves PR guidance to a dedicated file while keeping the template minimal. Content was faithfully moved and references are correctly updated.

Rules ❌ No requirements met

Repository Rules

❌ PR Description Quality Standards: Add concrete test commands to 'How did you test it?' and document actual risks in 'Potential risks' section (currently empty)

Options

Auto-apply is off → Gitar will not commit updates to this branch.
Display: compact → Showing less information.

Comment with these commands to change:

Auto-apply	Compact
gitar auto-apply:on	gitar display:verbose

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