chore: migrate repo references from juliusknorr to nextcloud org

[miaulalala]

Summary

• Replace all juliusknorr references with nextcloud across the codebase
• Updates GitHub repo URLs (github.com/juliusknorr/ → github.com/nextcloud/)
• Updates GitHub Pages docs URLs (juliusknorr.github.io/ → nextcloud.github.io/)
• Updates container image references (ghcr.io/juliusknorr/ → ghcr.io/nextcloud/)
• Updates the CI workflow repo guard condition so image builds publish to the new namespace

Note on container images

The ghcr.io/nextcloud/ images don't exist yet — they will be published automatically once the updated docker.yml workflow runs on this repo. A Nextcloud org admin will need to trigger or merge this and run a build to populate the new image namespace.

Test plan

• Verify grep -r juliusknorr . returns no results after merge
• Trigger the Docker build workflow to confirm images publish to ghcr.io/nextcloud/
• Confirm docs site is live at https://nextcloud.github.io/nextcloud-docker-dev/

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