USWDS-Site - Margin and padding: Add missing utilities

[amyleadem]

Summary

Updated the "Margin and padding" documentation to show the "medium-negative" margins that were added to the default set in #4212..

Specifically:

• Added tokens neg-4 through neg-15, and 10 through 15 to the example set because they exist in the USWDS margin defaults but were not included on the page.
• Changed the size of the example rectangles so that they could properly display the "medium-negative" margin sizes.
• Fixed the broken display on the examples in the "Padding on all sides" section

⚠️ Before merge, we must update the changelog data with the correct date.

Related issue

Closes #1927

Preview link

Preview link: Margin and padding utility page

Testing and review

Please confirm that all the "medium negative" margin utilities are accounted for in the documentation and that the visual examples make sense and are accurate.

[mejiaj]

Looks good, checked for code style and formatting and visual display issues.

[thisisdano]

Thank you!