docs: fix toc.not_included and trailing transition warnings

- Add rd_skl_drinking.ipynb to the Regression Discontinuity toctree;
  the notebook existed but was missing from the index, causing a
  toc.not_included WARNING.
- Remove trailing --- from estimands.md; a document-level transition
  (horizontal rule) as the last element is invalid in docutils and
  caused a WARNING: Document may not end with a transition.

Author: anevolbap

docs/source/knowledgebase/estimands.md

@@ -147,5 +147,3 @@ The descriptions above assume standard usage. Always consider what your specific
 | Regression Discontinuity | Local treatment effect at cutoff | Prediction-based |
 
 For methods not covered in detail here (IV, IPW, ANCOVA), see the respective notebook documentation, {doc}`quasi_dags` for identification, and the {doc}`glossary` for estimand definitions. Note that some of these methods have more limited implementation support in CausalPy---for example, IV does not yet have full `plot()` and `summary()` support, and IPW and ANCOVA are Bayesian-only.
-
----

docs/source/notebooks/index.md

@@ -70,6 +70,7 @@ piecewise_its_pymc.ipynb
 rd_skl.ipynb
 rd_pymc.ipynb
 rd_pymc_drinking.ipynb
+rd_skl_drinking.ipynb
 rd_donut_pymc.ipynb
 :::