docs: document dual-protocol (REST/gRPC) routing for Standard mode

## Summary

KServe v0.19 added dual-protocol (REST + gRPC) routing support for InferenceService Standard mode when using the Gateway API. No documentation exists for this feature.

## What needs to be documented

- How the HTTPRoute reconciler detects REST and gRPC ports from the backing Service
- gRPC detection: based on `appProtocol = kubernetes.io/h2c` and/or port name heuristics (containing grpc/h2c)
- Two HTTPRoute rules generated: gRPC match first (`application/grpc.*` content-type header), REST/HTTP fallback second
- Both protocols served behind the same hostname
- Why separate GRPCRoute + HTTPRoute for the same host doesn't work (Gateway API constraint)
- Example configuration showing a Service with both REST and gRPC ports

## Related PR

- https://github.com/kserve/kserve/pull/5451

## Since

KServe v0.19

Start with related PR #5451 and the website’s existing Gateway API documentation. Document REST and gRPC port detection, the ordered HTTPRoute matches, the shared-hostname constraint, and a Service example with both ports. Done means a newcomer can understand and configure dual-protocol routing in Standard mode.

Written by the indexing model from the issue text.