Update pandas.md

[SANAULLAH-AI]

docs: update pandas cheat sheet to v2.2+ with performance optimizations

• Add comprehensive sections: MultiIndex, categorical data, time series resampling, window functions, string operations
• Include performance & memory optimization techniques (PyArrow, copy-on-write, downcasting)
• Document deprecations: append() → pd.concat(), applymap() → map()
• Add version notes for pandas 2.0+ features
• Maintain two-column format: description | code for rapid lookup
• Include best practices for chained indexing, categorical dtypes, datetime parsing
• Add I/O formats: Parquet, Feather, HDF5, JSONL

[SANAULLAH-AI]

Updated Pandas.md