Add curated Reference Apps library

[robzolkos]

Adds a new /docs/reference-apps page — a hand-picked library of exemplary
open-source Rails apps, intended to show how real, production-quality Rails
applications are built.

What's included:

• New page at /docs/reference-apps with an image-led card grid
• Three real apps: Campfire, Writebook, and Fizzy (all by 37signals)
• Cards show a screenshot, author, name, blurb, and tag pills
• All content is data-driven via _data/reference_apps.yml — adding or
  editing an app requires no HTML changes
• Link card added to the /docs index page
• Six placeholder cards fill the grid while the library is curated;
  these are marked for removal before launch

Design notes:

• Cards stretch to equal height per row with tags pinned to the bottom
• Screenshot images are flush to card edges, clipped to rounded corners
• First card loads eagerly (LCP); subsequent cards are lazy-loaded