Refactor to be more in line with other platform operators

[razvan]

Description

As discussed here, the CRD of Spark applications should become more consistent with CRDs used by other Stackable operators.

### Tasks
- [ ] https://github.com/stackabletech/spark-k8s-operator/pull/277
- [x] Use resolved product image: https://github.com/stackabletech/spark-k8s-operator/pull/275
- [x] Make PySpark the only Spark image https://github.com/stackabletech/docker-images/pull/436
- [x] Update the docs