Add documentation for Request Routing in Kmesh (L7 capability)

[AkarshSahlot]

This PR contributes documentation for Issue https://github.com/kmesh-net/kmesh/issues/600, expanding the official Kmesh developer documentation with an example of Layer 7 (L7) request routing. Included in this PR: • A new file: docs/l7/request-routing.md • Introduction and overview of L7 request routing in Kmesh • Step-by-step example using Gateway API and httpbin-v1/httpbin-v2 services • YAML config for HTTPRoute with multiple match rules • Verification steps using curl from the sleep pod • Notes on prerequisites and known limitations • Related references for Kmesh and Istio documentation The goal is to help users understand and experiment with L7 routing based on path prefixes, and to build a foundation for future docs on additional L7 features (timeouts, traffic shifting, etc.). Please let me know if the structure or example should be adjusted. Happy to follow up with more L7 feature docs in subsequent PRs. Fixes https://github.com/kmesh-net/kmesh/issues/600

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