Consider making introduction page a standard form page

[dmethvin-gov]

There are several bits of special-case code related to handling the introduction page. There is also an implicit assumption that the route name for the page is "/introduction".

What if we just let people define form pages that could have only text (via a React component) and a "Continue" button? That allows not only a first-page introduction but interstitial help/continuation pages to introduce new chapters, and they can have any route name they want.

Related to usds/us-forms-system-starter-app#14 .

[jcmeloni-usds]

@dmethvin-gov Did this get resolved to y'all's satisfaction in subsequent issues/PRs?

[jcmeloni-usds]

Moved to phase 2.

[dmethvin-gov]

This ticket is blocked by #257