Add html::scroll-padding-top:80px to css

[Robadob]

Haven't tested this is actually the right place to inject the css rule, but should be fine (worst case it has no effect here).

Closes #841

[martinjohndyer]

Thanks @Robadob, that looks a lot better to me!

[ptheywood]

The fixed 80px value seems fine with current other CSS rules, but this will need updating if other css / logo images are made which impact the nav header.

Currently 2* 10px of padding from .header, 10px from .header .logo, and then I think the height of a h1 in the navbar (from h1 line-height: 1.2 and .header .logo font-size: 32px, plus something else?

Either way, its not based on css variables so an arbitrary absolute value would have to be used anyway, just we might have to adjust it in the future, and if users have their own accessibillity CSS it might not have the correct value.