docs: deployment.md points to ghcr.io/modernrelay/omnigraph-server, which requires auth; the public image is on Docker Hub

dremnik

[`docs/user/deployment.md`](https://github.com/ModernRelay/omnigraph/blob/main/docs/user/deployment.md#container) tells users to run `ghcr.io/modernrelay/omnigraph-server:<tag>`, but that package requires authentication: an anonymous pull token request returns `UNAUTHORIZED: authentication required`. The same image is public on Docker Hub as `docker.io/modernrelay/omnigraph-server` (tags through `v0.11.0` and `latest`, published alongside GHCR since #341). Either make the GHCR package public, or point the deployment guide at Docker Hub. Version: v0.11.0.

dremnik

Edited: the original version said no image had been published since v0.9.0. That was wrong. CI publishes the image for every tag, and v0.11.0 is on Docker Hub. The only remaining problem is that the documented GHCR path requires authentication.