Allow to tag books

[opatry]

A book can have several tags.
Need to decide if the tag should be a symbol or a string (or support both?)

• Add display label (and potentially other properties like an icon or emoji?) in nanoc.yaml for tags as symbol
• Adding a tags: field in the front matter
• Generate a page per /tags/xxx/
  • The page should display books for this tag, as a list of cards (like recent readings)
• Generate a /tags/ page listing all tags
• Display tags in book page (tag is clickable and redirect to /tags/xxx/)
• (Maybe?) Display tabs in book card
• Adjust view-transition.js to properly take into account these URLs (trying to make the pattern more generic)
• Update API accordingly
• Example of tags thriller, sci-fi, fantasy, policier, histoire, amour, nature, humour, …