Document python requirements install will not work in offline cluster

[maltesander]

We do a pip install in the requirements container

spark-k8s-operator/rust/operator-binary/src/spark_k8s_controller.rs

Line 185 in 2113e1c

format!("pip install --target={VOLUME_MOUNT_PATH_REQ} {req}"),

.

We should document that if pip requirements are given, this will not work in offline mode somewhere here https://github.com/stackabletech/spark-k8s-operator/blob/main/docs/modules/spark-k8s/pages/job_dependencies.adoc#spark-native-package-coordinates-and-python-requirements