Use rustdoc files again for error index page

[GuillaumeGomez]

#86503 made a small fix for the error index page, but it's still very likely that it'll break in no time because of it's not using rustdoc's style directly. Instead, we should keep specific style for this page in rust.css and use the "normal" rustdoc style for the rest.

[fee1-dead]

@rustbot claim