Evals docs (#7460)

Still don't have good "how to's", and the guides / examples section
could be further pruned and improved, but this PR adds a couple examples
for each of the common evaluator interfaces.

- [x] Example docs for each implemented evaluator
- [x] "how to make a custom evalutor" notebook for each low level APIs
(comparison, string, agent)
- [x] Move docs to modules area
- [x] Link to reference docs for more information
- [X] Still need to finish the evaluation index page
- ~[ ] Don't have good data generation section~
- ~[ ] Don't have good how to section for other common scenarios / FAQs
like regression testing, testing over similar inputs to measure
sensitivity, etc.~

docs/api_reference/modules/evaluation.rst

@@ -0,0 +1,9 @@
+Evaluation
+=======================
+
+LangChain has a number of convenient evaluation chains you can use off the shelf to grade your models' oupputs.
+
+.. automodule:: langchain.evaluation
+   :members:
+   :undoc-members:
+   :inherited-members: