Add pointer to fluidized bed code in examples

[andrewlee94]

A user recently asked where to find the code for the BFB models (IDAES/idaes-pse#1446). After checking the docs, it appears that it is not clear where to look for the actual code:

https://idaes-pse.readthedocs.io/en/stable/reference_guides/model_libraries/gas_solid_contactors/index.html

[bpaul4]

The page for the BFB unit model does include the class autodoc, which has the import location for the unit model: https://idaes-pse.readthedocs.io/en/stable/reference_guides/model_libraries/gas_solid_contactors/unit_models/bubbling_fluidized_bed.html#module-idaes.models_extra.gas_solid_contactors.unit_models.bubbling_fluidized_bed.

Checking some other pages, we do this for unit models, but not consistently for property packages or flowsheet models. It may be useful to ensure that all documented property and flowsheet models note the import path or have a link to the code.

[bpaul4]

Addressed by IDAES/idaes-pse#1451.