Write initial migration docs for carbon-components to @carbon/styles

[joshblack]

This issue is for updating the migration page available under docs/migration/v11.x-carbon-components.md to include the following migration information:

• Mapping filepaths from carbon-components to @carbon/styles
  • For example, src/globals/scss/_colors.scss is now at scss/_colors.scss
• Map common usage scenarios with carbon-components over to @carbon/styles
  • For example:
    • Here's what's changed in order to change the prefix
    • Here's what's changed in order to change the theme
    • In other words, here's the new way to do ___
  • Scenarios
    • Changing prefix
    • Specifying the theme
    • Passing in feature flags
    • Bringing in specific components
    • Utilities
    • Using stuff from vendor
• Document things in carbon-components that may still need to be implemented/migrated in @carbon/styles

Note: some of these will overlap with usage docs from: #9169

This will not include the following, as it is covered elsewhere:

• Design Token updates