Introduce horizontal navigation in NcAppNavigation

The problem:

Some apps that have too many columns. Some have 3, some have 4: NcAppNavigation, NcAppContentList, NcAppContent, and NcAppSidebar.

Here's an example in collectives. Keep in mind that although this is an extreme case, other apps might also suffer from displaying not so relevant content in lists and sidebar at the expense of user experience in interacting with the main content.

The proposed solution:

NcAppNavigation and NcAppContentList often represent a 2-dimensional list in which the user navigates to find the page that they want to open in the main content. It seems like in most use cases it's not necessary or important to show both lists at the same time, so I think a good solution to the problem is introducing a horizontal navigation within NcAppNavigation.

cc @nextcloud-libraries/designers

Begin by locating the NcAppNavigation and NcAppContentList components and reviewing how they are used together in the example described. The issue provides screenshots but no file paths, tests, or detailed interaction requirements; done means introducing horizontal navigation that reduces the need to display both lists simultaneously.