Description

Improve how we address breaking changes.

Give users the opportunity to fallback to previous behavior, but recommend and show how to implement the new behavior.

• Create setting for sidenav reorder
  • Default is FALSE for new behavior
  • Setting to TRUE for legacy implementations will revert to previous display and give warning.
  • Add SASS notification
• Update guidance
  • Update component guidance
  • Update settings page on site
  • Create changelog entry (for component & settings page)

From slack discussion 🔒 in #uswds

Additional information [optional]

Extension of work done in #4594.