docs: add Tuning Engines provider example

[cerebrixos]

OpenCode already supports custom OpenAI-compatible providers, but the provider docs do not include a concrete Tuning Engines example.

Adding a short example would make it clear how to configure:

• provider ID: tuning-engines
• base URL: https://api.tuningengines.com/v1
• API key format: sk-te-...
• model selection via a configured model alias

[github-actions]

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[github-actions]

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