docs: clarify Azure Realtime GA endpoint configuration

[OiPunk]

Summary

• add explicit Azure OpenAI realtime connection examples to docs using the GA endpoint format
• document both API key and bearer-token auth header variants in model_config
• warn against the legacy beta realtime URL format for realtime agents

Why

Issue: #1748 shows repeated confusion around Azure realtime URL/auth configuration and session.created validation errors caused by using legacy endpoint patterns.

Validation

• docs snippets added in:
  • docs/realtime/quickstart.md
  • docs/realtime/guide.md
• snippets align with existing RealtimeModelConfig fields (url, headers) used by realtime examples

Fixes #1748

[OiPunk]

Thanks for the review. I moved the Azure endpoint guidance to a dedicated bottom section in docs/realtime/guide.md (new section: "Azure OpenAI endpoint format").

Validation done locally before pushing:

• make build-docs (exit code 0)

Latest commit: bc457ea