Add custom estimator docs

docs/concepts/estimators.rst

@@ -9,6 +9,10 @@ or to rank ligands by affinity, we need absolute free energies (:math:`\Delta G`
 
 This requires an **estimator**: a method that takes the network of relative free energies and produces absolute values.
 
+Custom estimators can be added by subclassing :class:`cinnabar.estimators.Estimator` and passing an instance to
+:meth:`cinnabar.femap.FEMap.generate_absolute_values`. For a worked example, see the
+:doc:`custom estimator tutorial <../tutorials/custom-estimator>`.
+
 
 Maximum Likelihood Estimation (MLE)
 -----------------------------------

docs/tutorials/custom-estimator.ipynb

@@ -0,0 +1 @@
+../../examples/custom-estimator.ipynb

docs/tutorials/index.rst

@@ -14,6 +14,7 @@ API Tutorials
     :maxdepth: 1
 
     cinnabar-api
+    custom-estimator
     calculating-metrics
     ecdf-plots
     cycle-closure