docs(examples): add canonical OpenAI starter example under examples/openai

[kushalBanda]

OpenAI examples already exist across many topic folders, but there is no single canonical
starter in a dedicated examples/openai/ path. This makes first-time onboarding harder
because users must search across multiple example directories.

I’d like to add a minimal starter example at examples/openai/run.py that shows:

1. instructor.from_provider("openai/")
2. A small response_model
3. One basic .chat.completions.create(...) call

Scope is docs/examples only (no runtime behavior change).