fix(volcano): add missing PodGroup RBAC for kubectl-based installs (#800)

* fix(volcano): add missing PodGroup RBAC for kubectl-based installs

When LWS is installed via kubectl with Volcano gang scheduling enabled,
the LWS controller automatically creates scheduling.volcano.sh/podgroups
for each LWS replica. However, config/rbac/role.yaml is missing this
permission, causing PodGroup creation to silently fail.

Add config/components/volcano/ kustomize component (following the
existing config/components/prometheus/ pattern) and document the
required RBAC step in the gang-scheduling README for kubectl users.

The Helm chart already handles this conditionally via
gangSchedulingManagement.schedulerProvider=volcano.

Signed-off-by: hyukjin.jang <hyukjin.jang@navercorp.com>

* fix(volcano): use json6902 patch to append rules and update install docs

- Switch from strategic merge patch to JSON 6902 patch (op: add /rules/-)
  to avoid replacing the entire ClusterRole.rules array
- Add group/version to patch target for explicit matching
- Add commented [VOLCANO] entry in config/default/kustomization.yaml
  following the prometheus pattern (components must be included via parent)
- Update README to use config/default kustomization instead of direct
  component apply (Component kind cannot be applied standalone)

Signed-off-by: JangHyukJin <janghyukjin@navercorp.com>

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Signed-off-by: hyukjin.jang <hyukjin.jang@navercorp.com>
Signed-off-by: JangHyukJin <janghyukjin@navercorp.com>