doc: Support pygment syntax highlighting in PDF output

After some discussion on the developers' mailing list it was decided to not use colors for syntax highlighting in PDF documents. Instead, pygment's 'B/W' mode gets used, which needs bold, slanted, and bold-slanted typewriter shapes.

Texinfo output uses Computer Modern (CM) fonts. However, it doesn't provide bold typewriter (since the standard 'cmtt' shape is already quite heavy). For this reason we use Latin Modern (LM) fonts as a replacement, which provides (almost) the same glyph shapes but comes with a thinner typewriter default and thus have bold typewriter, too.