Use more readily available/open mono fonts

[cirosantilli]

At last I'm webpacking everything in my implementation, so I started with main.css as mentioned at #662 and now that I have it pretified I stated looking a the fonts: https://github.com/cirosantilli/node-express-sequelize-nextjs-realworld-example-app/blob/4d1e11d06c0b9fbbdfe5dddfb8f9c6376cfa48d4/demo.productionready.io.main.css I found all of them on the awesome https://github.com/fontsource/fontsource except one: none of the Mono fonts (used for markdown code blocks) are available in fontsource/google fonts. It would likely be a good idea to stick only to mono fonts present in either of those.

The relative CSS is:

code,
kbd,
pre,
samp {
  font-family: Menlo, Monaco, Consolas, liberation mono, courier new, monospace;
}