Add quickstart to OpenAI provider

[Lee-W]

What

Add a new quickstart.rst to the OpenAI provider docs: install → configure connection → write a first Dag, plus a "Where to go next" section pointing at the operators/connections reference and the vendor-agnostic common.ai provider.

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: [Calude] following the guidelines

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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.