[SPARK-53704] Rename pyspark-pi.yaml to pi-python.yaml

### What changes were proposed in this pull request?

This PR aims to rename `pyspark-pi.yaml` to `pi-python.yaml` for consistency.

### Why are the changes needed?

We use `appName` as the YAML file name. So, this should be `pi-python.yaml` for consistency.

https://github.com/apache/spark-kubernetes-operator/blob/cc1f475220fdca40b39a82877969590fd737c775/examples/pyspark-pi.yaml#L18

### Does this PR introduce _any_ user-facing change?

No behavior change.

### How was this patch tested?

Manual review.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes apache#340 from dongjoon-hyun/SPARK-53704.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun