Better document podOverrides in the Operators (and other small docs fixes)

[fhennig]

PodOverrides are only documented at the top level docs: https://docs.stackable.tech/home/stable/concepts/overrides#_pod_overrides but not in the Operators. We should reference the top level docs in the Operators to make it easier to find this feature for users if they are only reading the operator docs.

### Products
- [ ] https://github.com/stackabletech/airflow-operator/pull/513
- [ ] https://github.com/stackabletech/druid-operator/pull/630
- [ ] https://github.com/stackabletech/hbase-operator/pull/569
- [ ] https://github.com/stackabletech/hdfs-operator/pull/587
- [ ] https://github.com/stackabletech/hive-operator/pull/517
- [x] https://github.com/stackabletech/kafka-operator/pull/763
- [x] https://github.com/stackabletech/nifi-operator/pull/683
- [ ] https://github.com/stackabletech/spark-k8s-operator/pull/470
- [ ] https://github.com/stackabletech/superset-operator/pull/545
- [x] https://github.com/stackabletech/trino-operator/pull/649
- [x] https://github.com/stackabletech/zookeeper-operator/pull/865
- [x] https://github.com/stackabletech/opa-operator/pull/632

[maltesander]

all done!