Fix RealtimeModel reference (openai#1215)

## Summary - document `agents.realtime.model` so the RealtimeModel link works - include the new file in the documentation navigation ## Testing - `make format` - `make lint` - `make mypy` - `make tests` ------ https://chatgpt.com/codex/tasks/task_i_687fadfee88883219240b56e5abba76a

Author: rm-openai

docs/ref/realtime/model.md

@@ -0,0 +1,3 @@
+# `Model`
+
+::: agents.realtime.model

mkdocs.yml

@@ -125,6 +125,7 @@ plugins:
  - ref/realtime/session.md
  - ref/realtime/events.md
  - ref/realtime/config.md
+ - ref/realtime/model.md
  - Voice:
  - ref/voice/pipeline.md
  - ref/voice/workflow.md