docs: document Kubernetes Events on LLMInferenceService readiness transitions

## Summary

KServe v0.19 added Kubernetes Event emission on LLMInferenceService readiness state transitions. This provides push-based availability signals for operators and monitoring systems. No documentation exists for this feature.

## What needs to be documented

- `Normal` event with reason `LLMInferenceServiceReady` emitted on NotReady → Ready transition
- `Warning` event with reason `LLMInferenceServiceNotReady` emitted on Ready → NotReady transition (includes failing conditions list)
- `Warning` event with reason `UpdateFailed` on status update errors
- How operators can consume these events for alerting and monitoring
- Example `kubectl get events` output showing the events

## Related PR

- https://github.com/kserve/kserve/pull/5437

## Since

KServe v0.19

Start by reviewing the KServe v0.19 readiness transition behavior and related PR #5437, then locate the website section covering LLMInferenceService status or operations. Document the three event types, how operators can consume them for monitoring, and a representative kubectl get events example; done means all listed transitions and failure behavior are explained.

Written by the indexing model from the issue text.