Instrumental example notebook. (#555)

* Initial version of the instrumental example notebook.

* Updated the instrumental example notebook + index + CI

* Typo fixed

README.rst

@@ -125,6 +125,8 @@ approximations that power much of the modern probabilistic machine learning land
   - Causal inference with deep models and proxy variables
 - `Example: Mediation analysis and (in)direct effects <https://basisresearch.github.io/chirho/mediation.html>`_
   - Mediation analysis for path specific effects
+- `Example: Estimating causal effects using instrumental variables <https://basisresearch.github.io/chirho/instrumental_var.html>`_
+  - Causal effect estimation with instrumental variables
 - `Example: Deep structural causal model counterfactuals <https://basisresearch.github.io/chirho/deepscm.html>`_
   - Counterfactuals with normalizing flows
 - `Example: Structured Latent Confounders <https://basisresearch.github.io/chirho/slc.html>`_

docs/source/index.rst

@@ -27,6 +27,7 @@ Table of Contents
    deepscm
    slc
    mediation
+   instrumental_var
    sciplex
    sdid
    dr_learner