[Gap]: Document displayName on AI Gateway routes #1241

What needs documentation?

The displayName field on spec.routes[] of the AIGateway resource. The CRD accepts it, but the Model routing page lists only name, match.model and backendRefs, and the field doesn't appear on Providers and models either.

The CRD describes it as the human-readable label surfaced to model-discovery clients, such as Claude Code's model picker, through the display_name field of GET /v1/models. When it's empty, the operator falls back to the route's model id. The maximum length is 253 characters.

Context and references

The CRD field description on aigateways.ai-gateway.stacklok.dev (spec.routes[].displayName). The page needs a one-line mention in the route fields list and ideally a displayName in the YAML example, for instance:

routes:
  - name: claude-sonnet-5.5
    displayName: Claude Sonnet 5.5
    match:
      model: anthropic/claude-sonnet-5.5
    backendRefs:
      - provider: openrouter

Use case

Operators who expose models through the gateway want clients with a model picker to show readable names instead of raw slugs like anthropic/claude-sonnet-5.5. Without a mention in the docs, the field is only discoverable by reading the CRD.