Add Railway deployment guide

Covers deploying AnyCable-Go + Rails on Railway with:
- HTTP broadcasting over private networking (no Redis)
- JWT authentication for cross-domain WebSocket
- Turbo Streams + importmap-rails setup with vendored JS
- Presence indicator example
- Railway CLI commands alongside dashboard instructions
- Troubleshooting for common pitfalls (CSP, version pinning,
  signed stream verifier key load order, broadcast port)

Author: irinanazarova

docs/_sidebar.md

@@ -42,6 +42,7 @@
   * [Fly.io](/deployment/fly.md)
   * [Kamal](/deployment/kamal.md)
   * <a rel="noopener" href="https://github.com/anycable/thruster" target="_blank">Thruster 🔗</a>
+  * [Railway](/deployment/railway.md)
   * [Render](/deployment/render.md)
   * <a rel="noopener" href="https://hatchbox.relationkit.io/articles/12-does-hatchbox-support-anycable" target="_blank">Hatchbox 🔗</a>
   * [Docker](/deployment/docker.md)

docs/deployment/Readme.md

@@ -4,6 +4,7 @@ Check out the following guides for deploying AnyCable:
 
 * [Heroku](./heroku.md)
 * [Fly.io](./fly.md)
+* [Railway](./railway.md)
 * [Render](./render.md)
 * [Kubernetes](./kubernetes.md)
 * [Docker](docker.md)