Summary of the changes

Adds storybook playground for back to top component

Related issue

[x] Accessibility Insights FastPass performed.
[x] A11y unit test added and yields no issues.
[x] A11y plug-in on Storybook yields no issues.
[x] Manual screen reader testing performed using NVDA and VoiceOver.
[x] Manual keyboard testing for keyboard controls and logical focus order.
[x] Correct roles used and ARIA attributes used correctly where required.
[x] Logical heading structure is maintained, and the HTML elements used for headings can be changed to fit within the wider page structure.

[x] Page can be zoomed to 400% with no loss of content.
[x] Screen magnifier used with no issues.
[x] Text resized to 200% with no loss of content.
[x] Text spacing increased as per the WCAG 1.4.12 success criterion with no loss of content.

[x] Browser setting 'prefers reduced motion' tested. No animations or motion visible whilst this setting is on.
[x] Windows High Contrast mode tested with no loss of content.
[x] System light and dark mode tested with no loss of content.
[x] Browser support tested (Chrome, Safari, Firefox and Edge).

[x] Min/max content examples tested with no loss of content or overflow.
[x] All prop combinations work without issue.
[x] Tested for FOUC (Flash of Unstyled Content) in both SSR (Server-Side Rendering) and SSG (Static Site Generation) settings.
[x] Controlled and uncontrolled input components tested.