USWDS-Site - In-page nav: Revisit page content

[amyleadem]

Summary

The in-page navigation component page does not follow the structure of other component pages. We should review the content on this page and make edits to better align this component with other USWDS documentation.

Here are some of the discrepancies:

• Border is missing from the header. See example below.
  In-page nav (left) | Accordion (right)
  
• The "Using the in-page navigation component" section does not match the structure of other components. We should consider removing markup instructions and standardize the properties explanations.
• The page needs a review for plain language. Items like "root margin" and "threshold" feel particularly opaque.
• The bolded statements are not consistently complete sentences like on other pages. We should update these to be more meaningful.