Merged in feat/SW-3636-storybook-structure (pull request #3309)

feat(SW-3636): Storybook structure

* New sections in Storybook sidebar

* Group Storybook content files and add token files for spacing, border radius and shadows


Approved-by: Joakim Jäderberg

packages/design-system/.storybook/content/Spacing.mdx

@@ -0,0 +1,11 @@
+import { Meta } from '@storybook/addon-docs/blocks'
+
+import { Spacing } from './Spacing'
+
+import { base } from '../../lib/tokens'
+
+<Meta title="Tokens/Spacing" />
+
+# Spacing
+
+<Spacing theme={base} />