Update checkbox following UX review

Summary:

• Allow plain checkbox to be inline - especially since we are always using the plain checkbox
• Document that this component now followsDesignSystem
• Add examples with multiple checkboxes

To see the changes locally:

1. Checkout https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29324 locally and run the GDK
2. Navigate to localhost:3000 in chrome
3. Open the developer tools and go to the "Network" tab
4. Refresh the page
5. Find the application<Nonsense>.css network request and copy the URL - later referred to as <APP.CSS>
6. Navigate to 259-fix-plain-inline-checkboxes branch in gitlab-ui locally
7. Run CSS_URL=<APP.CSS> yarn storybook

Closes #259 (closed)