Refactor: Move AliasDataFrame to subdirectory

Structure:
- Move AliasDataFrame.py → AliasDataFrame/AliasDataFrame.py
- Move AliasDataFrameTest.py → AliasDataFrame/AliasDataFrameTest.py
- Add AliasDataFrame/__init__.py (maintains backward compatibility)
- Add AliasDataFrame/README.md
- Add AliasDataFrame/docs/ subdirectory
- Update dfextensions/__init__.py

Documentation:
- Add docs/COMPRESSION_GUIDE.md (comprehensive user guide)
- Add docs/CHANGELOG.md (version history)

Benefits:
- Consistent with other subprojects (groupby_regression/, quantile_fit_nd/)
- Self-contained subproject structure
- Clear documentation location
- Easy to add future features

Backward compatibility:
- All existing imports still work via updated __init__.py
- from dfextensions import AliasDataFrame
- from dfextensions.AliasDataFrame import CompressionState

Testing:
- All 61 tests still passing after restructure

Author: miranov25