Reduce right-side DOM size
This is another follow-up of https://github.com/rust-lang/rust/pull/100429 but not in code blocks this time.
So the idea is: if there is only one element in the `.rightside` element, there is no need to wrap it, we can just create one node.
On each page, I run this JS: `document.getElementsByTagName('*').length`. Important to note: the bigger the number of elements inside the page, the greater the gain. It also doesn't work very nicely on std docs because there are a lot of version annotations. So with this PR, It allows to get the following results:
| file name | before this PR | with this PR | diff |
|-|-|-|-|
| std/default/trait.Default.html | 2189 | 1331 | 39.2% |
| std/vec/struct.Vec.html | 14073 | 13842 | 1.7% |
| std/fmt/trait.Debug.html | 5313 | 4907 | 7.7% |
| std/ops/trait.Index.html | 642 | 630 | 1.9% |
| gtk4/WidgetExt | 3269 | 3061 | 6.4% |
You can test it [here](https://rustdoc.crud.net/imperio/reduce-rightsize-dom-size/gtk4/prelude/trait.WidgetExt.html).
r? `@notriddle`
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