GI triangles and claims reserving

Build this out with something more basic than https://institute-and-faculty-of-actuaries.github.io/mlr-blog/ but referencing perhaps https://chainladder-python.readthedocs.io/en/latest/

Start by comparing the requested GI triangles and claims reserving material with the referenced MLR blog and chainladder-python documentation. Define a simpler cookbook example and its scope first; done should be a complete, understandable treatment of the topic that is substantially more basic than the MLR material.

Written by the indexing model from the issue text.