docs: improve module index descriptions and update architecture overview

[HectorMozo3110]

This update improves the overall documentation structure by refining the description and context of each module, enhancing navigation, and updating the architecture overview to better reflect the current design of QML-HCS.

Documentation Update Checklist

• Updated the index.rst of each module with clearer descriptions and context
• Improved internal navigation and added consistent cross-links between modules
• Standardized the introduction sections across all module index pages
• Updated architecture.rst with corrected diagrams and contextual explanations
• Fixed outdated paths and ensured all Sphinx references resolve correctly
• Added bold + link style for module reference sections
• Validated that Sphinx builds without documentation reference warnings

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!