Remove duplicate doc.css anchor target styles, adjust hover styles

[sabine]

Since .prose is applied to the odoc output in the package documentation section, hover styles for the anchor elements in the headings were applied twice since #628. This patch removes the anchor target styles in doc.css, so that only one set of anchor target styles is applied.

Patch also changes the style of the hover effect because the # symbol would, on small screens, and when there are no sufficient margins, be cut off.

before	after
# is cut off at the edge of the browser window. In package docs, two # are rendered on hover	hover effect now requires just a very small margin and is applied only via .prose