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

All links inside paragraphs will have the same color as the anchor with
the name of the current module; they will also show up as underlined
when hovered over. This visually separates them from the rest of the
(black) content.

The name of the parent module found in the sidebar retains the
appearance like that of other links in the sidebar. Since it appears
inside of a `p` tag it explicitly removes any `text-decoration` on the
anchor.