Added Outputs and Serialization and Persistence user guides

[jbednar]

Added two new user guides:

• Serialization and Persistence: Saving the state of a Parameterized object to a text, script, or pickle file
• Outputs: Output types and connecting output to Parameter inputs

There was also planned to be a Customization user guide, but after writing it there was very little material, so that material was added to the Parameters user guide instead.

Other misc changes:

• Added basic coverage of pydantic in Comparisons
• Fixed MultiFileSelector example in Parameter_Types. Added classlist and descendents docs.
• Documented edit_constant, shared_parameters, and pprint in Parameters