Rollup merge of #91534 - jsha:heading-color, r=GuillaumeGomez

Make rustdoc headings black, and markdown blue

Demo:

https://rustdoc.crud.net/jsha/heading-color/std/string/index.html#structs
https://rustdoc.crud.net/jsha/heading-color/std/string/struct.String.html#examples

Fixes #91304

r? ```@camelid``` /cc ```@GuillaumeGomez```

(Note: we may want to make rustdoc headings and markdown headings the same color -- #90245 -- but we would want to do that intentionally; this is fixing up a change that did so accidentally)

src/librustdoc/html/render/print_item.rs

@@ -296,7 +296,7 @@ fn item_module(w: &mut Buffer, cx: &Context<'_>, item: &clean::Item, items: &[cl
             let (short, name) = item_ty_to_strs(myty.unwrap());
             write!(
                 w,
-                "<h2 id=\"{id}\" class=\"section-header\">\
+                "<h2 id=\"{id}\" class=\"small-section-header\">\
                     <a href=\"#{id}\">{name}</a>\
                  </h2>\n{}",
                 ITEM_TABLE_OPEN,

src/librustdoc/html/static/css/themes/ayu.css

@@ -219,7 +219,7 @@ a {
 a.srclink,
 a#toggle-all-docs,
 a.anchor,
-.section-header a,
+.small-section-header a,
 #source-sidebar a,
 pre.rust a,
 .sidebar a,

src/librustdoc/html/static/css/themes/dark.css

@@ -181,7 +181,7 @@ a {
 a.srclink,
 a#toggle-all-docs,
 a.anchor,
-.section-header a,
+.small-section-header a,
 #source-sidebar a,
 pre.rust a,
 .sidebar a,

src/librustdoc/html/static/css/themes/light.css

@@ -176,7 +176,7 @@ a {
 a.srclink,
 a#toggle-all-docs,
 a.anchor,
-.section-header a,
+.small-section-header a,
 #source-sidebar a,
 pre.rust a,
 .sidebar a,