Rollup merge of #85396 - jsha:top-doc-font-sizes, r=GuillaumeGomez
Rollup merge of #85396 - jsha:top-doc-font-sizes, r=GuillaumeGomez

rustdoc: restore header sizes

The `<details>` toggle work changed the relationship from #main to the top-doc docblock so it was no longer a parent relationship but a great-grandparent relationship. This updates the CSS rules that set the heading sizes so they still apply.

Fixes #85389

r? ``@camelid``

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust