Update guide for on-scroll-header

[antonjuulnaber]

Hi

The guide on making a scrolling header is great, but it relies on javascript running every time you scroll. That is really expensive (resource-wise) and contributes to a laggy an unresponsive website.

I think the guide should be updated to use sticky css-elements instead of js. This method does not have the same performance issues, and I'd argue it's an easier and more elegant solution. The new guide is pretty much already written here.

The support for sticky elements is not perfect, so there is a discussion to be had about whether to prioritize browser support or performance, but i think at the very least the page should have a yellow box, mentioning that the sticky element is a better solution.