[Docs] Add pull request template + review checklist

[JasonEran]

Epic: #8

Goal

Make PRs easy to review and aligned with release quality.

Acceptance Criteria

• PR template asks for linked issue, validation, and rollback notes.
• Review checklist covers tests, security, docs.

[JasonEran]

Final Evidence

Completed by PR #51:

• [v2.3][Process] Complete templates + Mermaid class diagrams #51
• Merge commit: 3ea5247

Acceptance mapping:

• PR template now asks for linked issue, validation details, and rollback notes.
• Review checklist now explicitly includes tests, security impact, and docs.