Add a way to quickly scroll back to the top on long pages

[stasadev]

It would be nice to have an easy way to go back to the top when reading long pages, for example:

https://ddev.com/blog/markdown-features-demo/#featured-sponsors

A small floating "Back to Top" control is a common solution, but any simple way to jump back to the top would work.

This would make long blog posts and other long pages easier to navigate.