Enable Spark Operator by default

[kunal-511]

Pull Request Template for Kubeflow Manifests

✏️ Summary of Changes

Added Spark Operator to the example/kustomization.yaml resources list, enabling it by default.
Updated README.md to include Spark Operator (version 2.1.1) in the component versions table.
Added an installation section for the Spark Operator in the individual components guide.

✅ Contributor Checklist

• I have tested these changes with kustomize. See Installation Prerequisites.
• All commits are signed-off to satisfy the DCO check.
• I have considered adding my company to the adopters page to support Kubeflow and help the community, since I expect help from the community for my issue (see 1. and 2.).

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[juliusvonkohout]

/lgtm
/approve

[google-oss-prow]

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