docs: add reference recipes and tutorials for common routing patterns

Summary

Add maintained reference recipes and end-to-end tutorials for common vLLM Semantic Router deployment patterns so adopters can start from known-good baselines instead of assembling configs from scattered examples.

Parent roadmap: #2287 Related: #2326, #2330, #2331, #2294 Related focused issue: #2378 for out-of-the-box preset tuning across balance, cost-first, accuracy-first, speed-first, privacy-first, and grounding-first.

Motivation

Adopters need clear production-facing examples for common use cases. Current documentation explains pieces of the system, but there is not yet a maintained set of reference configurations that can be copied, validated, and adapted.

Scope

Create tutorials and reference configs for at least these patterns:

• complexity routing;
• model/provider failover;
• PII and guardrail routing;
• multi-model selection;
• custom signal extension;
• recipe/entrypoint based virtual models;
• agent-oriented routing and learning diagnostics when available.

Each reference should include:

• a complete config or recipe;
• expected request examples;
• expected routing behavior;
• validation commands;
• known limitations and production caveats.

Non-goals

• Do not add new runtime features only to make examples work.
• Do not use docs to hide missing validation or broken config surfaces.

Acceptance criteria

• Maintained reference configs exist under the repo's recipe/config surfaces.
• Website tutorials explain the supported path for each pattern.
• Examples pass the relevant config validation gate.
• The docs link to the related roadmap issues for features that are still experimental.

Validation

make agent-report ENV=cpu CHANGED_FILES="config deploy/recipes website/docs"
make agent-lint CHANGED_FILES="config deploy/recipes website/docs"
make agent-ci-gate CHANGED_FILES="config deploy/recipes website/docs"