feat: add Roj experiment section

[Lukafin]

Summary

• Add a new Experiments nav item and homepage section
• Add a Roj card linking to https://roj.world
• Include the experiment in AI-mode markdown for English and German

Test Plan

• HTML smoke check confirms the experiments section and roj.world link
• Previewed locally and captured screenshot
• Jekyll build not run: Ruby/Bundler/Jekyll are not installed in this environment