feat(deployment): add envFrom support for ConfigMap/Secret environment variables

Add support for loading environment variables from ConfigMap or Secret
resources using envFrom field, following Kubernetes best practices for
configuration management.

Changes:
- Add jellyfin.envFrom parameter in values.yaml
- Add envFrom section in deployment.yaml template (before env)
- Provide comprehensive documentation with examples
- Bump chart version 2.5.0 -> 2.6.0 (minor - new feature)
- Add changelog annotation for Artifact Hub

Use cases:
- Load multiple environment variables from ConfigMap without defining each one
- Load secrets from external secret management systems
- Separate configuration from chart values (12-factor app pattern)
- Simplify configuration for complex deployments

Example usage:
  jellyfin:
    envFrom:
      - configMapRef:
          name: jellyfin-config
      - secretRef:
          name: jellyfin-secrets

Follows standard Kubernetes patterns used by most community Helm charts.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Aleksei Sviridkin <f@lex.la>

charts/jellyfin/values.yaml

@@ -191,7 +191,20 @@ jellyfin:
   command: []
   # -- Additional arguments for the entrypoint command.
   args: []
+  # -- Load environment variables from ConfigMap or Secret.
+  # See: https://kubernetes.io/docs/tasks/configure-pod-container/configure-pod-configmap/#configure-all-key-value-pairs-in-a-configmap-as-container-environment-variables
+  # Example:
+  # envFrom:
+  #   - configMapRef:
+  #       name: jellyfin-config
+  #   - secretRef:
+  #       name: jellyfin-secrets
+  envFrom: []
   # -- Additional environment variables for the container.
+  # Example:
+  # env:
+  #   - name: JELLYFIN_CACHE_DIR
+  #     value: /cache
   env: []
 
 persistence: