[Feature] Add a scroll-to-top element to UX for longer page contexts

## Description

Many pages within Creative Commons websites have much longer pages, in these cases a `scroll-to-top` element would serve a valid purpose in certain contexts to aid in improved UX.

The element would likely not be present on smaller screens which already have some measure of functionality related to this baked into their operating system level UX; but for larger viewport sizes this could be much more helpful to include.

## Additional context
* https://github.com/creativecommons/cc-legal-tools-app/issues/474
* https://github.com/creativecommons/cc-resource-archive/pull/216

Solutions here should explore non-js interventions. CSS pseudo elements using z-index stack positions and variable percentage based heights would offer a means to "reveal" the button after a degree of scrolling, and to visually hide the element on pages which do not reach a particular length. Special care for keyboard accessibility should be taken under consideration to ensure that it can receive appropriate focus and order handling.

## Implementation

- [x] I would be interested in implementing this feature.