USWDS - Docs: Add usage instructions to component settings.

[mejiaj]

Description

Closes #1017 and #1221. This adds descriptions to component settings.

Preview links

Settings page →
Component pages →

Additional information

• Uses consistent punctuation for Variants and Settings
• Fixes broken references to some Component subsections
• Removes duplicate keys
• Checks setting exists before creating <code> in settings table
• Corrects package references (Related: USWDS - Create process list package →)
• Adds links to guidance and package info on Footer

Before you hit Submit, make sure you’ve done whichever of these applies to you:

• Follow the 18F Front End Coding Style Guide and Accessibility Guide.
• Run npm test and make sure the tests for the files you have changed have passed.
• Run your code through HTML_CodeSniffer and make sure it’s error free.
• Title your pull request using this format: [Website] - [UI component]: Brief statement describing what this pull request solves.

[thisisdano]

Let's merge this even though I'm not sure the trailing period is the right punctuation. There are other good changes in here and at least this is consistent