docs(862): new design overview page

[lhaggar]

closes #862

What

1. Background - Update the design overview page with the latest content
2. The design overview page should have the latest content

I have done:

• [ ] Written unit tests against changes
• [ ] Written functional tests against the component and/or NewsKit site
• [ ] Updated relevant documentation

I have tested manually:

• [ ] The feature's functionality is working as expected on Chrome, Firefox, Safari and Edge
• [ ] The screen reader reads and flows through the elements as expected.
• [ ] There are no new errors in the browser console coming from this PR.
• [ ] When visual test is not added, it renders correctly on different browsers and mobile viewports (Safari, Firefox, small mobile viewport, tablet)
• [ ] The Playground feature is working as expected

Before:

After:

Who should review this PR:

How to test:

[pp-serviceaccount]

You can preview these changes on: Documentation Site: http://ncu-newskit-docs-pr.s3-website-eu-west-1.amazonaws.com/862-update-design-overview-page Storybook: http://ncu-newskit-docs-pr.s3-website-eu-west-1.amazonaws.com/862-update-design-overview-page/storybook