Migrate extension nav group to individual nav items about podman-desktop

[benoitf]

Before implementing anything globally, it needs to be validated by the implementation for extensions.

As if Kubernetes is moved to an extension, it should behave the same. And then I'm not sure it can behave in the same way depending on how it's implemented

[deboer-tim]

Aside from AI Lab (and possibly Kubernetes in the future), no other extension contributes more than one 'page'. I'd prefer to use #7759/#7761 to figure out if we keep the concept of nav groups and whether extensions can (easily) provide them, and use this issue to move single-page extensions 'out' of the nav section (and using the same registry as built-in pages) so that we could start supporting things like show/hide or handling too many items.