Storybook: Remove "experimental" category in favor of a badge

[tyxla]

In #66275 we're currently in the process of reorganizing the components by categories.

However, we separate components by stable and experimental already, and as a result, we get multiple directories with the same category:

Stable:

Experimental:

Ideally, we shouldn't have that multi-level distinction by category, and the categories would be at the top level.

To achieve that, I think we can mark the experimental components with a badge, just like we currently mark WIP or private components.

cc @WordPress/gutenberg-components

[ciampo]

Related conversation in #66197