Update Documentation to use new tutorial.py module for grabbing catalog URLs

[jukent]

We now have curated small catalogs in the tutorial-catalogs directory from #454 and #455 that can be called via intake_esm.tutorial.get_url(). The documentation should be updated to use this method for some demonstrations.

In the same effort, the documentation can be updated to use "catalog" consistently (without misnamed references to "collections").