rustdoc: #13771: Make html links inside paragraphs more readable

All links inside docblocks will have their color set to `#4e8bca` (a
light blue color to contrast against the black text). This color also
offers a visible contrast from the surrounding text if viewed as
grayscale, making it suitable for accessability.

Docblock links will also be underlined when hovered over.

src/librustdoc/html/static/main.css

@@ -202,6 +202,10 @@ nav.sub {
     white-space: nowrap;
     text-overflow: ellipsis;
 }
+
+.docblock a { color: #4e8bca; }
+.docblock a:hover { text-decoration: underline; }
+
 .docblock.short p {
     overflow: hidden;
     text-overflow: ellipsis;