CSS: fonts: prefer Menlo to Monaco everywhere

For monospaced fonts, Monaco is one of the very first fonts released with
the Mac.
With OS X 10.6, Apple came out with a new, improved font called Menlo
which took over Monaco as the system's default monospaced font.

Besides, Menlo has smaller glyphs that line up better with other common
"console" fonts and make more sense for today's screens.
(We did hit that problem at mozilla/rust#10330.)

Note: Menlo was already used as default in docs.css