docs: Document resource parsing with examples

[adwk67]

Description

Document the mapping of CRD resource definitions onto spark conf settings. Fixes https://github.com/stackabletech/spark-k8s-operator/issues/294

Also removed spark.kubernetes.{executor|driver}.{request|limit}.memory as it does not appear in the spark code/branch-3.4.

~CI: https://ci.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/spark-k8s-operator-it-custom/106/~ CI: :green_circle: https://ci.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/spark-k8s-operator-it-custom/108/

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

# Author
- [x] Helm chart can be installed and deployed operator works
- [x] Integration tests passed (for non trivial changes)

# Reviewer
- [ ] Code contains useful comments
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated
- [ ] Changelog updated
- [ ] Cargo.toml only contains references to git tags (not specific commits or branches)

# Acceptance
- [ ] Feature Tracker has been updated
- [ ] Proper release label has been added

[adwk67]

Jenkins: 🟢 ci.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/spark-k8s-operator-it-custom/108