docs: add Python 3.11 ReadTheDocs environment file

Add environment-py311-rtd.yml for building docs with Python 3.11
and updated dependency versions:
- scipy>=1.16.0
- numpy>=1.25.2
- scikit-learn>=1.6.0
- statsmodels>=0.14.5
- cython==3.0.11

Author: jeongyoonlee

docs/environment-py311-rtd.yml

@@ -0,0 +1,31 @@
+name: causalml-rtd-py311
+channels:
+  - conda-forge
+  - defaults
+dependencies:
+  - pip=24.0
+  - python=3.11
+  - pandoc
+  - sphinx
+  - sphinx_rtd_theme
+  - sphinxcontrib-bibtex<2.0.0
+  - nbsphinx
+  - pip:
+    - cython==3.0.11
+    - dill==0.3.8
+    - importlib-metadata==8.5.0
+    - joblib==1.4.0
+    - lightgbm==4.5.0
+    - matplotlib==3.9.2
+    - multiprocess==0.70.16
+    - numba==0.60.0
+    - numpy>=1.25.2
+    - pandas==2.2.2
+    - pyro-api==0.1.2
+    - pyro-ppl==1.9.1
+    - scikit-learn>=1.6.0
+    - scipy>=1.16.0
+    - seaborn==0.13.2
+    - shap==0.46.0
+    - statsmodels>=0.14.5
+    - xgboost==2.1.3