CSG-1097: Update Comet Starter to better support theming

[jbouder]

Updates required to allow the Comet Starter App to utilize USWDS design tokens for customizing styling.

Description

• Removed USWDS css from index exports
• Removed Header and Footer components from library in favor of using custom header and footer provided through the app
• Updated Storybook welcome page with info on the Comet Starter App
• Removed unneeded icon dependency

Related Issue

N/A

Motivation and Context

• The current method of bundling of USWDS css as part of the comet package breaks the ability to utilize USWDS design tokens. In order to resolve this, the following is required: 1. Remove css export from index file. 2. Add USWDS styling to the comet starter app through the provided SCSS packages.

How Has This Been Tested?

• This has been tested by creating a beta version of this branch, then successfully adding to the comet starter app and configuring as needed.

Screenshots (if appropriate):

[LuisdelaVega]

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