docs: add vLLM Semantic Router integration

[danehans]

Summary

• add a focused vLLM Semantic Router integration overview
• distinguish vSR from the vLLM inference provider integration
• link to the upstream deployment guide and agentgateway cost-routing example
• add a shared architecture diagram with explicit external-processing and backend-routing boundaries
• rename the page slug to vllm-semantic-router while preserving vllm as an alias

Validation

• git diff --check
• xmllint --noout assets/img/integrations/vllm-semantic-router-cost-routing.svg
• verified both external resource links return HTTP 200
• hugo160 --gc --minify

[cloudflare-workers-and-pages]

Deploying agentproxy with    Cloudflare Pages

Latest commit:	a9a3263
Status:	✅  Deploy successful!
Preview URL:	https://022fc8e0.agentproxy.pages.dev
Branch Preview URL:	https://docs-vllm-semantic-router-in.agentproxy.pages.dev

View logs