Improve the consumer getting started for repos

The next natural step that pure consumers want is to locate and install different packages from different repositories.

This is a bit hidden in the docs, maybe it would be good to extend the "getting started" section or in the next one, to introduce consumers to these cases.

Start by reading the existing getting started section and the adjacent documentation described in the issue. Add an introduction to locating and installing packages from different repositories for pure consumers, and consider whether it belongs in that section or the next one; done means those cases are no longer hidden.

Written by the indexing model from the issue text.