Auto merge of #90186 - jsha:fix-header-sizes, r=GuillaumeGomez

Fix documentation header sizes

And add a rustdoc-gui test confirming various header sizes.

Split off from #90156. This fixes a regression in #89506 where the heading level of titles within Markdown was too high (h2) for docblocks under structs, unions, and enum impls.

r? `@camelid`

Demo: https://jacob.hoffman-andrews.com/rust/fix-header-sizes/std/string/struct.String.html#impl-Add%3C%26%27_%20str%3E
Stable: https://doc.rust-lang.org/stable/std/string/struct.String.html#impl-Add%3C%26%27_%20str%3E
Beta: https://doc.rust-lang.org/beta/std/string/struct.String.html#impl-Add%3C%26%27_%20str%3E

src/librustdoc/html/render/mod.rs

@@ -1612,7 +1612,7 @@ fn render_impl(
                     error_codes: cx.shared.codes,
                     edition: cx.shared.edition(),
                     playground: &cx.shared.playground,
-                    heading_offset: HeadingOffset::H2
+                    heading_offset: HeadingOffset::H4
                 }
                 .into_string()
             );