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 curated 37signals apps: Campfire, Writebook, and Fizzy
• 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

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

Verification:

• ruby -ryaml -e 'data=YAML.load_file("_data/reference_apps.yml"); abort "expected 3 apps" unless data.fetch("apps").size == 3; abort "mock screenshots remain" if data.fetch("apps").any? { |app| app["screenshot"].to_s.include?("mock-") }; puts "✓ reference_apps.yml has exactly 3 non-mock apps"'
• BUNDLE_USER_HOME=/tmp/rails-website-bundle-home BUNDLE_PATH=/tmp/rails-website-bundle bundle exec jekyll build

[dhh]

Is there anything else we need for this to go live?

[rafaelfranca]

We need to remove all the placeholders and then we can merge this.

[dhh]

Would be nice if the Study the reference apps block was centered instead of left aligned. Feels like it's dangling a little. Otherwise good.

[robzolkos]

@dhh done

[dhh]

Oh ha. I thought we actually had more examples than just the three from 37s! My mistake 😄. But yes, let's just go with this anyway for now if this is what we have. Hopefully we'll soon see more we can add.