Add additional usage examples to ModelLibrary documentation

[emolter]

Implementation of ModelLibrary on the JWST side revealed some common use-cases and coding patterns that would be useful to document in the AbstractModelLibrary docs. These were:

• example workflow where map_function() is used as a generator non-trivially. i.e., for data in lib.map_function(lambda m, i: m.data, modify=False): # do something read-only with the data for this model

• example that shows usage of list(lib.map_function()) to modify models in place

• library shelve with try…except…finally when errors might be raised while models are open (and note that the try…except should be entirely inside the with context, otherwise finally will not see the library as open)

• link to documentation of asn dictionaries, e.g. this one for JWST. For this point, it would eventually be useful to agree on a common standard for associations between Roman and JWST, and perhaps move some common association code to stpipe.