Add Deploy on Railway documentation page

[douglasrubims]

Summary

• Add a new deployment/deploy-on-railway page with the Railway 1-click deploy button
• Register the page in docs.json under the Build section, following the same pattern as Deploy on DigitalOcean

Test plan

• Run the docs site locally and confirm "Deploy on Railway" appears in the Build navigation
• Open /deployment/deploy-on-railway and verify the page renders correctly
• Click the Railway deploy button and confirm it opens the Chartbrew template on Railway