Normalize CHANGELOG.md structure

[nanotaboada]

Problem

CHANGELOG.md is missing two elements present in all other repos: reference links at the bottom and the release template comment.

Proposed Solution

Apply the following additions to CHANGELOG.md:

1. Add the <!-- Template for new releases: --> comment block after the last release entry and before the reference links. Use the standard template shared across all repos:

<!-- Template for new releases:

## [X.Y.Z - CLUB_NAME] - YYYY-MM-DD

### Added
- New features

### Changed
- Changes in existing functionality

### Deprecated
- Soon-to-be removed features

### Removed
- Removed features

### Fixed
- Bug fixes

### Security
- Security vulnerability fixes

-->

2. Add reference links at the very bottom of the file (after the template comment), following the pattern used in all other repos:

[unreleased]: https://github.com/nanotaboada/java.samples.spring.boot/compare/v2.0.1-chelsea...HEAD
[2.0.1 - Chelsea]: https://github.com/nanotaboada/java.samples.spring.boot/compare/v2.0.0-barcelona...v2.0.1-chelsea
[2.0.0 - Barcelona]: https://github.com/nanotaboada/java.samples.spring.boot/compare/v1.0.0-arsenal...v2.0.0-barcelona
[1.0.0 - Arsenal]: https://github.com/nanotaboada/java.samples.spring.boot/releases/tag/v1.0.0-arsenal

Acceptance Criteria

• Template comment added before the reference links
• Reference links added at the bottom for all four releases plus [Unreleased]
• No other content changed

[coderabbitai]

🔗 Related PRs

#287 - ci(release): implement named releases with tag-based deployment (#252) [merged]
#291 - docs(changelog): prepare release notes for v1.0.0-arsenal [merged]
#297 - docs: normalize README structure and add RELEASES.md [merged]
#305 - docs(changelog): prepare release notes for v2.0.0-barcelona [merged]
#316 - docs(changelog): prepare release notes for v2.0.1-chelsea [merged]

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