Feature: Improved PR-Issue Linking and Association Model

Improve the model for associating PRs and issues in release note generation:
- Allow weak linking between PRs and issues (not just strong/closing links)
- Merge PR and Issue label/grouping logic
- User control over where release notes content appears (description, comments, etc.)

Deliverables:
- Configurable association/linking logic
- UI/UX to help users select linking mode
- Backward compatibility and clear migration guidance
- Documentation for all association strategies

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**Related sub-issues:**
- #96 Merge PR and issue labels when considering release notes group
- #79 Introduce weak link between PR and Issue
- #77 Provide support for different locations of release notes