Address PR #410 R1 P2: restore shadowed function inventory

Reviewer flagged that page-level :no-index: on .. autofunction:: (added
in initial PR 2.5 commit to resolve 3 page-vs-page duplicates) made
diff_diff.{trop,stacked_did,chaisemartin_dhaultfoeuille} convenience
functions absent from the py:function inventory entirely - so :py:func:
cross-refs and intersphinx pointers to those public functions wouldn't
resolve.

Empirical investigation surfaced the actual conflict source: each of the 3
files had a top-level ``.. module:: diff_diff.<name>`` directive that
registered the module FQN, colliding with the function FQN of the same
name. Removing the .. module:: directive lets .. autofunction:: register
the function as the canonical (and only) :py:function entry. The
.. autoclass:: directives on those pages were already converted to
top-level paths (diff_diff.X form) in the initial PR 2.5 commit and
don't depend on the implicit module context.

Verified: make html shows 0 ERRORs / 0 WARNINGs / 0 duplicates; all 3
convenience functions present in objects.inv as py:function entries.
Doc-snippet test parity preserved (111 passed, 4 skipped).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

docs/api/chaisemartin_dhaultfoeuille.rst

@@ -100,8 +100,6 @@ library option for non-absorbing treatments.
   Difference-in-Differences Estimators of Intertemporal Treatment
   Effects. NBER Working Paper 29873.
 
-.. module:: diff_diff.chaisemartin_dhaultfoeuille
-
 ChaisemartinDHaultfoeuille
 --------------------------
 
@@ -157,7 +155,6 @@ Convenience Function
 --------------------
 
 .. autofunction:: diff_diff.chaisemartin_dhaultfoeuille
-   :no-index:
 
 Standalone TWFE Decomposition Diagnostic
 ----------------------------------------

docs/api/stacked_did.rst

@@ -23,8 +23,6 @@ which addresses bias in naive stacked DiD regressions by:
 Difference-in-Differences. *NBER Working Paper* 32054.
 `<http://www.nber.org/papers/w32054>`_
 
-.. module:: diff_diff.stacked_did
-
 StackedDiD
 ----------
 
@@ -68,7 +66,6 @@ Convenience Function
 --------------------
 
 .. autofunction:: diff_diff.stacked_did
-   :no-index:
 
 Example Usage
 -------------

docs/api/trop.rst

@@ -25,8 +25,6 @@ which combines three robustness components:
 **Reference:** Athey, S., Imbens, G. W., Qu, Z., & Viviano, D. (2025). Triply Robust
 Panel Estimators. *Working Paper*. `arXiv:2508.21536 <https://arxiv.org/abs/2508.21536>`_
 
-.. module:: diff_diff.trop
-
 TROP
 ----
 
@@ -74,7 +72,6 @@ Convenience Function
 --------------------
 
 .. autofunction:: diff_diff.trop
-   :no-index:
 
 Tuning Parameters
 -----------------