Rollup merge of #91102 - jsha:theme-anchor, r=GuillaumeGomez

Set color for <a> in a more straightforward way.

Previously, we set the default color for <a> tags to black, and then had an override with a bunch of not() clauses to set anchors in
docblocks to blue.

Instead, we should set the default color for <a> to blue (or equivalent in other themes), and override it for places like the sidebar or search results, where we don't want them to be styled as links.

Demo at https://rustdoc.crud.net/jsha/theme-anchor/std/string/struct.String.html. This should result in no visible changes.

r? `@GuillaumeGomez`

src/test/rustdoc-gui/anchors.goml

@@ -0,0 +1,17 @@
+goto: file://|DOC_PATH|/test_docs/struct.HeavilyDocumentedStruct.html
+
+// Set the theme to light.
+local-storage: {"rustdoc-theme": "light", "rustdoc-use-system-theme": "false"}
+// We reload the page so the local storage settings are being used.
+reload:
+
+assert-css: ("#toggle-all-docs", {"color": "rgba(0, 0, 0, 0)"})
+assert-css: (".fqn .in-band a:nth-of-type(1)", {"color": "rgba(0, 0, 0, 0)"})
+assert-css: (".fqn .in-band a:nth-of-type(2)", {"color": "rgba(0, 0, 0, 0)"})
+assert-css: (".srclink", {"color": "rgba(0, 0, 0, 0)"})
+assert-css: (".srclink", {"color": "rgba(0, 0, 0, 0)"})
+
+assert-css: ("#top-doc-prose-title", {"color": "rgba(0, 0, 0, 0)"})
+
+assert-css: (".sidebar a", {"color": "rgba(0, 0, 0, 0)"})
+assert-css: (".in-band a", {"color": "rgba(0, 0, 0, 0)"})