rustdoc: use CSS containment to speed up render

rustdoc: use CSS containment to speed up render

https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Containment

This affected layout a little and required adjustments to the CSS to
keep spacing the same. In particular, the margins of adjacent items
usually overlap with each other. However, when an item has contain:
layout, any margins of child nodes push out the size of the item itself.
This was making spacing between items a little too big. To solve that, I
removed margins in some places: in particular for certain classes that
often occur at the end of a `details.rustdoc-toggle` block, I removed
their bottom margin. Generally, the margins provided by the next item
down are sufficient.

Also remove an unnecessary margin-top on .code-header.