docs: clarify container runtime requirements for ImageVolume (cloudnative-pg#9368)

Add explicit container runtime version requirements to the ImageVolume
documentation. While the `ImageVolume` feature gate must be enabled in
Kubernetes 1.33, users also need a compatible container runtime version:

- containerd v2.1.0 or later
- CRI-O v1.31 or later

This addresses issues where users enable the feature gate but encounter
errors due to older container runtime versions that don't support
ImageVolume (e.g., containerd 1.7.27 in default minikube images).

Closes cloudnative-pg#9354

Signed-off-by: Marco Nenciarini <marco.nenciarini@enterprisedb.com>
Signed-off-by: Gabriele Bartolini <gabriele.bartolini@enterprisedb.com>
Co-authored-by: Gabriele Bartolini <gabriele.bartolini@enterprisedb.com>

Author: mnencia

docs/src/imagevolume_extensions.md

@@ -48,6 +48,9 @@ To use image volume extensions with CloudNativePG, you need:
 
 - **PostgreSQL 18 or later**, with support for `extension_control_path`.
 - **Kubernetes 1.33**, with the `ImageVolume` feature gate enabled.
+- **Container runtime with `ImageVolume` support**:
+    - `containerd` v2.1.0 or later, or
+    - `CRI-O` v1.31 or later.
 - **CloudNativePG-compatible extension container images**, ensuring:
     - Matching PostgreSQL major version of the `Cluster` resource.
     - Compatible operating system distribution of the `Cluster` resource.