Update pre/code font-family to include `monospace` as fallback

Without monospace as a fallback, code blocks on doc.gitlab.com are not using monospace fonts. They look ugly as a result, if you don't have the listed fonts installed (or others defined as alternatives). While the fonts are great, I don't think any of them are installed by default on Ubuntu, but it looks fine because DejavuSans Mono is defined as an alternative to one of those (presumably Lucida Console). However, on distros like Arch, where this definition isn't present, we get to see them in sans. 😢 It doesn't hurt to keep monospace as a fallback.