Docs: configuration.rst

[ceholden]

Documentation for model parameter exploration guide page docs/guide/configuration.rst

https://jacobian.org/writing/great-documentation/

[ceholden]

This should be an up to date version of more or less what was contained within:
https://github.com/ceholden/yatsm/blob/ebcc4701f260f32b3bc57caf38d230a827eece40/docs/guide/configuration.rst

[ceholden]

Add information about how YATSM deals with pickled estimators ("built in" and user provided) and passing of additional keyword arguments to sklearn_estimator.fit and other methods. See #82