Rollup merge of #93576 - jsha:fix-rustdoc-html, r=GuillaumeGomez

Emit more valid HTML from rustdoc

Previously, tidy-html5 (`tidy`) would complain about a few things in our HTML. The main thing is that `<summary>` tags can't contain `<div>`s. That's easily fixed by changing out the `<div>`s for `<span>`s with `display: block`.

However, there's also a rule that `<span>`s can't contain heading elements. `<span>` permits only "phrasing content" https://developer.mozilla.org/en-US/docs/Web/HTML/Element/span, and `<h3>` (and friends) are "Flow content, heading content, palpable content". https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Heading_Elements

We have a wrapping `<div>` that goes around each `<h3>`/`<h4>`, etc. We turn that into a `<section>` rather than a `<span>` because `<section>` permits "flow content". https://developer.mozilla.org/en-US/docs/Web/HTML/Element/section

After this change we get only three warnings from tidy, run on struct.String.html:

line 6 column 10790 - Warning: trimming empty <span>
line 1 column 1118 - Warning: <link> proprietary attribute "disabled"
line 1 column 1193 - Warning: <link> proprietary attribute "disabled"

The empty `<span>` is a known issue - there's a span in front of the search box to work around a strange Safari issue.

The `<link>` attributes are the non-default stylesheets. We can probably refactor theme application to avoid using this proprietary "disabled" attribute.

We can suppress those warnings with flags to tidy, and get a run that returns 0 (success):

```
tidy -o /dev/null -quiet --drop-empty-elements no --warn-proprietary-attributes no build/x86_64-unknown-linux-gnu/doc/std/string/trait.ToString.html
```

Note: this requires the latest version of tidy-html5, built from https://github.com/htacg/tidy-html5. Older versions (including the default version on Ubuntu 21.10) think `<section>` can't occur inside `<summary>`.

Demo: https://rustdoc.crud.net/jsha/fix-rustdoc-html/std/string/struct.String.html

r? `@GuillaumeGomez`

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