feat: create groupby_regression package structure

- Add package __init__.py with version info
- Create tests/ subdirectory with __init__.py
- Create benchmarks/ subdirectory with __init__.py
- Create docs/ and benchmark_results/ directories
- Prepare for file migration with git mv

Imports in __init__.py commented out until files are moved.

Author: miranov25

UTILS/dfextensions/groupby_regression/__init__.py

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+"""
+GroupBy Regression Package
+
+Provides two implementations:
+- Robust (groupby_regression.py): Production-proven, full features, custom fitters
+- Optimized (groupby_regression_optimized.py): Speed-optimized (v2/v3/v4)
+
+Quick Start:
+    # Robust implementation (battle-tested)
+    from dfextensions.groupby_regression import GroupByRegressor
+    _, dfGB = GroupByRegressor.make_parallel_fit(...)
+    
+    # Fast implementation (17-200× faster)
+    from dfextensions.groupby_regression import make_parallel_fit_v4
+    _, dfGB = make_parallel_fit_v4(...)
+
+See docs/README.md for choosing between implementations.
+"""
+
+# Import main classes from modules (will add after files are moved)
+# from .groupby_regression import GroupByRegressor
+# from .groupby_regression_optimized import (
+#     make_parallel_fit_v2,
+#     make_parallel_fit_v3,
+#     make_parallel_fit_v4,
+#     GroupByRegressorOptimized,
+# )
+
+# Version info
+__version__ = '2.0.0'
+__author__ = 'Marian Ivanov'
+
+# Expose at package level (will uncomment after files are moved)
+# __all__ = [
+#     'GroupByRegressor',
+#     'make_parallel_fit_v2',
+#     'make_parallel_fit_v3',
+#     'make_parallel_fit_v4',
+#     'GroupByRegressorOptimized',
+# ]

UTILS/dfextensions/groupby_regression/benchmarks/__init__.py

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+"""Benchmark suite for groupby_regression package"""

UTILS/dfextensions/groupby_regression/tests/__init__.py

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+"""Test suite for groupby_regression package"""