docs: add tutorial for progressive delivery on custom StatefulSet-like workloads

[abhisheksainimitawa]

What

Adds docs/tutorials/custom_workload.md — a step-by-step guide documenting support for progressive delivery on custom CRDs that follow the StatefulSet update model.

Also adds a Docs entry in CHANGELOG.md under v0.6.2.

Closes #242.

Why

The rollouts.kruise.io/workload-type: statefulset label has been supported in the codebase for a while (see pkg/util/constant.go, pkg/webhook/workload/mutating/unified_update_handler.go) but was never documented. Users with custom operators (Elasticsearch, Kafka, custom GameServer CRDs, etc.) that use a StatefulSet-style partition update model have no way to discover this capability.

What the doc covers

Section	Content
How it works	Webhook intercepts update, sets partition=MaxInt32, Rollout controller advances partition per batch
CRD requirements	spec.replicas, spec.template, spec.updateStrategy.rollingUpdate.partition must exist
Step 1	Add rollouts.kruise.io/workload-type: statefulset label to the custom workload
Step 2	Create a Rollout with workloadRef pointing to the custom CRD
Step 3	Trigger rollout by updating Pod template; approve steps via kubectl-kruise
Step 4	Rollback by reverting the template
Troubleshooting	Common failure modes with concrete checks

How I verified the content

The doc was written by reading the actual implementation:

• pkg/util/constant.go — WorkloadTypeLabel and StatefulSetType constants
• pkg/webhook/workload/mutating/unified_update_handler.go — how workload-type triggers webhook interception
• pkg/util/parse_utils.go — IsStatefulSetRollingUpdate and SetStatefulSetPartition (shows the partition mechanism and CRD field requirements)
• pkg/controller/rollout/rollout_controller.go — patchWorkloadRolloutWebhookLabel (how the webhook is dynamically configured)

Signed-off-by: abhisheksainimitawa abhi9813293405@gmail.com

/cc @furykerry @ls-2018

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[kruise-bot]

Welcome @abhisheksainimitawa! It looks like this is your first PR to openkruise/rollouts 🎉

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.38%. Comparing base (b2600e9) to head (63f81ee).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

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  Files          66       66           
  Lines        8559     8559           
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  Hits         4398     4398           
  Misses       3575     3575           
  Partials      586      586           

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