Specify a default font

[fabi1cazenave]

There’s no specific font-face yet for the website, which makes it inconsistent between the normal and monospaced fonts — the latter being used for key IDs and single characters.

More specifically, some fonts don’t handle combining diacritics well (see #125) and we kinda need a good default font to ensure our SVG keyboards render properly.

Maybe Fira could be a good candidate to start ?