[SPARK-60116] Document Kueue default LocalQueue as not supported

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

This PR aims to add a row to the Kueue feature support table of `docs/spark_custom_resources.md` to document that the default `LocalQueue` of Kueue is not supported.

### Why are the changes needed?

For the built-in integrations, Kueue sets the `kueue.x-k8s.io/queue-name` label of a job without the label to `default` if its namespace has a `LocalQueue` named `default`. This does not apply to `SparkApplication` and `SparkCluster`, since the Kueue webhooks do not cover the `spark.apache.org` group. So the label should be set explicitly, e.g. `kueue.x-k8s.io/queue-name: default`.

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

No. This is a documentation-only change for an unreleased feature.

### How was this patch tested?

Manual review against the code and Kueue v0.20.0.

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

Generated-by: Claude Opus 5.5

Closes #961 from dongjoon-hyun/SPARK-60116.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: dongjoon-hyun

docs/spark_custom_resources.md

@@ -678,6 +678,7 @@ relates it to the reconciliation.
 | MultiKueue                                                                | Not supported       | `SparkApplication` and `SparkCluster` have no `spec.managedBy` field, which MultiKueue needs to run them on a worker cluster.                                                                                                                |
 | Elastic workloads (`Workload` slices)                                     | Not supported       | A resource has one `Workload` whose pod set counts are fixed.                                                                                                                                                                                |
 | `manageJobsWithoutQueueName` and `managedJobsNamespaceSelector`           | Not supported       | Kueue does not manage the Spark custom resources itself, so a resource without the `kueue.x-k8s.io/queue-name` label is not queued and starts right away.                                                                                    |
+| Defaulting the queue name to the `default` `LocalQueue` of the namespace  | Not supported       | A resource without the `kueue.x-k8s.io/queue-name` label is not queued even if its namespace has a `LocalQueue` named `default`, so set the label explicitly, e.g. `kueue.x-k8s.io/queue-name: default`.                                     |
 | Dynamic allocation (`spark.dynamicAllocation.enabled`)                    | Not supported       | Such a `SparkApplication` fails with `SchedulingFailure` instead of being queued.                                                                                                                                                            |
 | `SparkCluster` with `minWorkers < maxWorkers`                             | Not supported       | Such a `SparkCluster` fails with `SchedulingFailure` instead of being queued.                                                                                                                                                                |
 | Pod template files (`spark.kubernetes.{driver,executor}.podTemplateFile`) | Not supported       | A `SparkApplication` whose pod template is set only in such a file fails with `SchedulingFailure`, so set it in the [spec](#pod-template-support) instead.                                                                                   |