[docs] Reorganize Storybook component navigation

[suzubara]

Does this documentation relate to a specific USWDS feature?
No

Is your documentation related to a problem or inconsistency? Please describe.
Most of the components in this library map directly to a USWDS component, but some of them are composed of multiple components, or are Truss-designed components (and should maybe be moved into a different library).

Describe the documentation you'd like
Update the Storybook sidebar navigation so that it is as easy as possible to find USWDS components (consider following the same structure as https://designsystem.digital.gov/components/), and organize Truss & other components separately.