docs: Add OpenAI API compatibility page

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What does this PR do?

This adds some initial content documenting our OpenAI compatible APIs

• Responses, Chat Completions, Completions, and Models - along with instructions on how to use them via OpenAI or Llama Stack clients and some simple examples for each.

It's not a lot of content, but it's a start so that users have some idea how to get going as we continue to work on these APIs.

Test Plan

I generated the docs site locally and verified things render properly. I also ran each code example to ensure it works as expected. And, I asked my AI code assistant to do a quick spell-check and review of the docs and it didn't flag any obvious errors.