docs: add changelog to documentation site

[sunt05]

Summary

• Added CHANGELOG.md to the documentation site for better visibility of recent changes
• Created new changelog.rst page that includes the main CHANGELOG.md file
• Integrated changelog into documentation navigation and index page
• Added comprehensive Kiro specification for current and future enhancements

Implementation Details

• Used Sphinx's literalinclude directive to include CHANGELOG.md directly
• Maintains single source of truth - no duplicate content
• Added to "For developers/contributors" section in navigation
• Added reference link in main index page for easy discovery

Kiro Specification

Complete specification added in .claude/specs/add-changelog-to-docs/:

• requirements.md: 24 requirements in EARS notation
• design.md: Technical architecture including future automation
• tasks.md: 26 implementation tasks across 6 phases
• tag-system-proposal.md: Proposal for user/dev changelog filtering

Testing

• Verified changelog.rst syntax is correct
• Confirmed navigation links are properly added
• Documentation builds successfully (requires local build environment)

Future Enhancements

As detailed in the specification:

• Automatic changelog updates via CI/CD
• Filtered views for user-facing vs developer changes
• Integration with release automation
• Improved tagging system ([user:new], [dev:new], etc.)

Fixes #537

Test plan

• Documentation builds without errors
• Changelog page displays correctly
• Navigation links work properly
• CHANGELOG.md content is properly rendered