Rollup merge of #128615 - notriddle:notriddle/anchor-a11y, r=GuillaumeGomez

rustdoc: make the hover trail for doc anchors a bit bigger

https://rust-lang.zulipchat.com/#narrow/stream/266220-t-rustdoc/topic/Weird.20markdown.20heading.20rendering.3F

r? ```@GuillaumeGomez```

### Screenshots (the purple part is the padding, which the mouse can pass through to hover over it)

| Before | After |
|--|--|
| ![image](https://github.com/user-attachments/assets/5070eebe-84ac-4f5d-8950-58664370191c) | ![image](https://github.com/user-attachments/assets/55340161-ad8a-41bd-a120-8cdab11f4af0)

Author: matthiaskrgr