Add the possibility to form itineraries out of different Guides

**Is your feature request related to a problem? Please describe.**
At the moment it is only possible to create guides without context to other guides, except you name them in a specific way.

**Describe the solution you'd like**
A cool way to handle that would be to be able to form a wrapper around different guides and call it itineraries. That way you could create multiple guides for one powershell module for example or you could, more generally speaking, form topics out of guides.

Start by examining how PSAtlas currently represents Guides and uses the existing guide-naming convention. Clarify how an itinerary should group multiple Guides, including Guides for one PowerShell module, and define the expected behavior for creating and viewing topic-based collections.

Written by the indexing model from the issue text.