docs: Add documentation for LLMInferenceService label and annotation propagation

[khushiiagrawal]

Describe the change you'd like to see

Add a documentation page for the LLMInferenceService label and annotation propagation feature, as requested by @sivanantha321 in kserve/kserve#5009 (kserve/kserve#5009 (comment)).

This feature (merged in kserve/kserve#5009) enables users to propagate Kubernetes labels and annotations from an LLMInferenceService resource to the workload pods it manages. It supports all deployment modes: single-node Deployments, multi-node LeaderWorkerSets, disaggregated prefill-decode workloads, and the scheduler (EPP) Deployment.

The documentation should cover:

• Two propagation layers: top-level metadata (prefix-filtered via an approved allowlist) vs. spec-level fields (spec.labels, spec.annotations, and per-component equivalents) which propagate all keys without filtering.
• Approved prefix allowlists: which annotation prefixes (k8s.v1.cni.cncf.io, kueue.x-k8s.io, prometheus.io) and label prefixes (kueue.x-k8s.io) are propagated from .metadata.
• Per-component spec fields: spec.prefill.labels/spec.prefill.annotations for prefill pods and spec.router.scheduler.labels/spec.router.scheduler.annotations for the scheduler pod.
• Multi-node behaviour: propagation to both leader and worker pod templates.
• Precedence rules: spec-level values override top-level metadata when the same key appears in both.
• Practical examples: Kueue queue assignment, Multus CNI attachment, Prometheus scraping config, and custom platform labels for cost allocation.

Suggested location: docs/model-serving/generative-inference/llmisvc/llmisvc-label-propagation.md under the existing LLMInferenceService sidebar category.

Additional context