Change Tree in kickstartDS Storybook

[DanielLeyUX]

Please clean up our main Storybook tree to match a proper Component Module Structure (as we have in RUB)

Content
please remove appearence, Showcases, Glossary and move these into a new group called Custom

[julrich]

Most importantly, the categories should be ordered like this:

1. Welcome
2. Design Token (we should remove the plural "Design Tokens" used currently)
3. Base
4. Blog
5. Content
6. Form
7. Custom
8. Recipes
9. Pages

[fleven-kds]

Pushed on master with the following order:

[julrich]

I'd vote for re-ordering the entries of the Design Token-category, too.

1. Colors
2. Spacing
3. Typography
4. Box Shadow
5. Border Radius

This emphasizes important categories, and is nicer to look at as the entry to the section.