Switch to Jekyll

[chastell]

Fixes #303.
Fixes #246.

Notable changes:

• uses the github-pages gem to set up automagic GitHub Pages deploys (at the cost of older Jekyll, but it’s worth it),
• redirects from / to /en (rather than special-casing the English version),
• switches fonts to Merriweather & Montserrat (from Google Fonts) to support non-English diacritics and other alphabets (like Cyrillic),
• combines source/localizable/_code-of-conduct.xx.md and locales/xx.yml files into a single xx.md file for easier translation.

Let me know what you think. 😅