docs: add OCI registry caveat docs

Signed-off-by: Xe Iaso <me@xeiaso.net>

docs/docs/admin/roles/_category_.json

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+{
+  "label": "Server Roles",
+  "position": 40,
+  "link": {
+    "type": "generated-index",
+    "description": "Various server roles you will need to keep in mind with Anubis."
+  }
+}

docs/docs/admin/roles/oci-registry.mdx

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+# OCI Registries
+
+If you are serving an OCI registry behind Anubis, you will need to import the `(data)/clients/docker-client.yaml` file in order to make sure that OCI registry clients can download images:
+
+```yaml
+bots:
+  - import: (data)/meta/default-config.yaml
+  - import: (data)/clients/docker-client.yaml
+# ... the rest of your config
+```