docs: Add usage guide on Kubernetes executors

[sbernauer]

Description

Fixes #370 (all grep -r 'workers:' findings fixed)

Previously it was only at https://docs.stackable.tech/home/stable/airflow/ and IMHO hard to find

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

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1. Changes are OpenShift compatible
2. CRD changes approved
3. CRD documentation for all fields, following the style guide.
4. Helm chart can be installed and deployed operator works
5. Integration tests passed (for non trivial changes)
6. Changes need to be "offline" compatible

Reviewer

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1. Code contains useful comments
2. (Integration-)Test cases added
3. Documentation added or updated. Follows the style guide.
4. Changelog updated
5. Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

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1. Feature Tracker has been updated
2. Proper release label has been added

[adwk67]

LGTM - thanks!