Describe what a private catalog is

[zmitchell]

In the "Packages and Catalog" concept page, we should describe what a private catalog is now that we'll have the ability for customers to publish packages to them.

Base your branch off of the build-preview branch and submit your PR against the same branch.

[stahnma]

A Flox Catalog is the collection of packages and associated metadata that can be consumed via a Flox Environment.

A Custom Catalog is a collection of those packages and metadata for your organization's software.

/cc @limeytexan

[limeytexan]

A Flox Catalog is a collection of package artifacts and associated metadata that can be consumed via a Flox Environment.

There are two types of catalogs:

1. The Base Catalog is populated by Flox and contains packages as maintained by the Nix Community by way of The Nixpkgs Collection
2. Custom Catalogs are maintained by the Users and Organizations that own them by way of the flox publish command, as described in the [Build and Publish](insert publish documentation URL here) reference

The visibility of Custom Catalogs can be public or private, and packages from all types of catalog are consumed by way of the same flox (search|show|install) commands.