Anchor offset doesn't work on the mobile view

[vjeux]

In order to get #links to work correctly with the static header, we offset it by the size of the header + some free room. But the mobile view has another header, which is not taken into account. So when you click on the link, it doesn't jump to the right location.

https://github.com/facebookexperimental/Docusaurus/blob/f72df94dd33f4d92f151a78a52a4f9733ddf2991/lib/static/css/main.css#L78-L82

[hramos]

Since I'm already touching this in #164 (which I've confirmed has the same issue), I'll just fix this issue there as well.