Rollup merge of #102253 - jsha:css-contain, r=notriddle

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.

In particular this helps with the problem that rustdoc in some situations can generate giant HTML pages, which can crash a Chrome tab on typical modern hardware, for instance: `https://docs.rs/iced-x86/1.16.0/iced_x86/code_asm/struct.CodeAssembler.html` (26MB, 409k DOM nodes). This doesn't, of course, universally solve the problem, but it pushes out the boundary of the largest page rustdoc can produce without crashing a browser tab.

Demos:

https://rustdoc.crud.net/jsha/css-contain/std/string/struct.String.html
(warning: giant page, _may_ crash a browser tab) https://rustdoc.crud.net/jsha/css-contain-icedx86/iced_x86/code_asm/struct.CodeAssembler.html

r? `@notriddle`

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