Change some typography in the docs.

Two issues are addressed with this commit:

1. Some <pre> tags were inheriting the "Serif" font on Linux, causing
"code"-ish stuff to appear with proportional-width font instead of
monospaced-width font.
2. Font stack with "Helvetica Neue" ... has been changed to default to
sans-serif instead of "Serif", this produces a more consistent and
friendlier look on Windows in particular.
Doc fix: replace "hugolibs" with "hugo" in "contributing" page, fixes #271.