Add "back to top" links

For online docs, and especially for long pages, it would be useful with a "back to top" link at the end of the page. This is also recommended by the MS writing style guide: https://learn.microsoft.com/en-us/style-guide/scannable-content/#include-navigation-within-long-documents

Example: https://dev.epicgames.com/documentation/en-us/unreal-engine/hardware-and-software-specifications-for-unreal-engine