Update documentation for estimators module refactoring

- README.md: Use `from diff_diff import TwoWayFixedEffects` (preferred)
- docs/api/estimators.rst: Document new module structure (twfe.py, synthetic_did.py)
  with backward-compatible import instructions

Author: claude

README.md

@@ -539,7 +539,7 @@ Works with `DifferenceInDifferences` and `TwoWayFixedEffects` estimators.
 ### Two-Way Fixed Effects (Panel Data)
 
 ```python
-from diff_diff.estimators import TwoWayFixedEffects
+from diff_diff import TwoWayFixedEffects
 
 twfe = TwoWayFixedEffects()
 results = twfe.fit(

docs/api/estimators.rst

@@ -3,6 +3,20 @@ Estimators
 
 Core estimator classes for Difference-in-Differences analysis.
 
+The main estimators module (``diff_diff.estimators``) contains the base classes
+``DifferenceInDifferences`` and ``MultiPeriodDiD``. Additional estimators are
+organized in separate modules for maintainability:
+
+- ``diff_diff.twfe`` - ``TwoWayFixedEffects`` estimator
+- ``diff_diff.synthetic_did`` - ``SyntheticDiD`` estimator
+
+All estimators are re-exported from ``diff_diff.estimators`` and ``diff_diff``
+for backward compatibility, so you can import any of them using:
+
+.. code-block:: python
+
+    from diff_diff import DifferenceInDifferences, TwoWayFixedEffects, MultiPeriodDiD, SyntheticDiD
+
 .. module:: diff_diff.estimators
 
 DifferenceInDifferences
@@ -24,23 +38,25 @@ Basic 2x2 DiD estimator.
       ~DifferenceInDifferences.get_params
       ~DifferenceInDifferences.set_params
 
-TwoWayFixedEffects
-------------------
+MultiPeriodDiD
+--------------
 
-Panel DiD with unit and time fixed effects.
+Event study estimator with period-specific treatment effects.
 
-.. autoclass:: diff_diff.TwoWayFixedEffects
+.. autoclass:: diff_diff.MultiPeriodDiD
    :members:
    :undoc-members:
    :show-inheritance:
    :inherited-members:
 
-MultiPeriodDiD
---------------
+TwoWayFixedEffects
+------------------
 
-Event study estimator with period-specific treatment effects.
+Panel DiD with unit and time fixed effects.
 
-.. autoclass:: diff_diff.MultiPeriodDiD
+.. module:: diff_diff.twfe
+
+.. autoclass:: diff_diff.TwoWayFixedEffects
    :members:
    :undoc-members:
    :show-inheritance:
@@ -51,6 +67,8 @@ SyntheticDiD
 
 Synthetic control combined with DiD (Arkhangelsky et al. 2021).
 
+.. module:: diff_diff.synthetic_did
+
 .. autoclass:: diff_diff.SyntheticDiD
    :members:
    :undoc-members: