Rollup merge of #100775 - GuillaumeGomez:reduce-span-v2, r=notriddle

rustdoc: Merge source code pages HTML elements together v2

This is the follow-up of https://github.com/rust-lang/rust/pull/100429.

I strongly recommend to review it one commit at a time because otherwise it's a lot at once.

For these ones, on each page, I run this JS: `document.getElementsByTagName('*').length`. The goal is to count the number of DOM elements. I took some pages that seemed big, but don't hesitate to check some others. I also added the "starting point" because it's quite nice to see how much the page was reduced thanks to these two PRs.

| file name | before #100429 | before this PR | with this PR | diff |
|-|-|-|-|-|
| std/lib.rs.html (source link on std crate page) | 3455 | 2332 | 1772 | 24% |
| alloc/vec/mod.rs.html (source on Vec type page) | 11012 | 5982 | 5833 | 2.5% |
| alloc/string.rs.html (source on String type page) | 10800 | 6010 | 5822 | 3.2% |
| std/sync/mutex.rs.html (source on Mutex type page) | 2953 | 2041 | 2038 | 0.1% |

So unsurprisingly, the more attributes you have, the bigger the difference.

You can test it [here](https://rustdoc.crud.net/imperio/reduce-span-v2/src/std/lib.rs.html).

cc ``````@jsha``````
r? ``````@notriddle``````

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