Rollup merge of #91480 - jsha:fewer-colors, r=GuillaumeGomez

rustdoc: use smaller number of colors to distinguish items

This reduces visual distractions when reading method signatures.

As discussed in https://github.com/rust-lang/rust/issues/59845#issuecomment-974757191, this categorizes items into one of six colors (down from thirteen):

 - method, function (ochre `#AD7C37`)
 - trait, trait alias (dark slate blue `#6E4FC9`)
 - enum, struct, type alias, union, primitive (maroon `#AD378A`)
 - static, module, keyword, associated type, foreign type (steel blue `#3873AD`)
 - macro (green `#068000`)
 - generic params, self, Self (unmarked black `#000000`)

I slightly tweaked the actual color values so they'd have the same lightness (previously the trait color stood out much more than the others). And I made the color for links in general consistently use steel blue (previously there was a slightly different color for "search-failed").

The ayu and dark themes have been updated according to the same logic. I haven't changed any of the color values in those themes, just their assignment to types.

Demo:

https://rustdoc.crud.net/jsha/fewer-colors/std/string/struct.String.html
https://rustdoc.crud.net/jsha/fewer-colors/std/vec/struct.Vec.html
https://rustdoc.crud.net/jsha/fewer-colors/std/io/trait.Read.html
https://rustdoc.crud.net/jsha/fewer-colors/std/iter/trait.Iterator.html

src/librustdoc/html/render/mod.rs

@@ -788,7 +788,7 @@ fn assoc_type(
 ) {
     write!(
         w,
-        "{}type <a href=\"{}\" class=\"type\">{}</a>",
+        "{}type <a href=\"{}\" class=\"associatedtype\">{}</a>",
         extra,
         naive_assoc_href(it, link, cx),
         it.name.as_ref().unwrap()