Add Athlytics to Open Source Projects

[HzaCode]

This adds Athlytics to the Open Source Projects section.

Why it's awesome:

• Academic R toolkit specifically designed for endurance training analytics
• Works 100% offline with local Strava data exports (privacy-focused)
• Implements evidence-based sports science metrics (ACWR, Efficiency Factor, Pa:Hr Decoupling)
• Published on CRAN with comprehensive documentation
• Built for cohort analysis and multi-athlete comparisons

Links:

• GitHub: https://github.com/HzaCode/Athlytics
• Documentation: https://hezhiang.com/Athlytics/
• CRAN: https://cran.r-project.org/package=Athlytics