document available case datasets in the package

[SpatLyu]

This issue tracks and documents all built-in case datasets included in the package for reproducibility and user reference.

Currently includes:

• Abundance time-series data of two unicellular ciliates Paramecium aurelia and Didinium nasutum (PR new abundance case dataset #70)
• Greater Perth traffic crash dataset (PR add Greater Perth traffic crash dataset as a case study  #60)
• Loess Plateau precipitation–soil moisture–NDVI dataset (PR include case datasets #41)

This issue serves as a central place to maintain dataset descriptions, metadata, and usage examples.