BUG: What is a "catalog"? Documentation does not answer.

[prjemian]

The documentation starts to use the term catalog without ever describing what such a thing is. So, it is a BUG. Various examples of catalogs are described but in place is there a definition of this term.

Such a fundamental term deserves its own documentation page.

As a plus, this would be a useful anchor for facilities to use as a reference for teaching their users about this fundamental term.