Simplify and generalize navigation

[aduth]

🛠 Summary of changes

Updates how side navigation is generated within the local documentation site, to avoid tying it specifically to components.

This is part of a series of changes looking to introduce new sections for "Utilities" in addition to components, which will need to have its own side navigation.

📜 Testing Plan

The only meaningful change is that visiting the root URL will redirect automatically to the first component.

1. npm start
2. Visit http://localhost:4000/
3. Observe that you are redirected to http://localhost:4000/accordions/

[aduth]

This implementation is not new, it already existed in the now-removed uswds.rb. This is the only utility from that module we still needed, and the naming "USWDS" was confusing, so I renamed the file and module to reflect the single remaining helper.

[aduth]

Visual regression tests caught a change that I wasn't intending, which is that previously the side navigation would "expand" and show sub-headings of a page, e.g. "Button sizes and states" on the Buttons page. This no longer happens. I think I'm fine with this side-effect and we could consider the In-Page Navigation component if we wanted something like this in the future, but good to highlight another instance of the visual regression tests doing their job.

Before	After