[SPARK-38189][K8S][DOC] Add `Priority scheduling` doc for Spark on K8S
[SPARK-38189][K8S][DOC] Add `Priority scheduling` doc for Spark on K8S

### What changes were proposed in this pull request?
Document how to set the priority class with the pod template.

### Why are the changes needed?
Currently, we didn't have a certain doc to help user enable priority scheduling

Related: https://github.com/apache/spark/pull/35716 https://github.com/apache/spark/pull/35639#issuecomment-1055847723

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
![image](https://user-images.githubusercontent.com/1736354/156696247-cf2cf566-57a0-4b8a-a18f-aa300a6f6a3d.png)

Closes #35728 from Yikun/SPARK-38189-doc.

Authored-by: Yikun Jiang <2206089eaa6cfa02a44178196e5767f367cbca91@gmail.com>
Signed-off-by: Dongjoon Hyun <6ff48b33cbb828151035af58a92f18a39f1d1036@apache.org>
(cherry picked from commit 727f044612c0a71097aa0d29cb3f24a53b93fc1f)
Signed-off-by: Dongjoon Hyun <6ff48b33cbb828151035af58a92f18a39f1d1036@apache.org>

milliman/spark,milliman/spark,milliman/spark,milliman/spark,milliman/spark,milliman/spark,milliman/spark