docs: clarify model config IDs

[849261680]

Issue for this PR

Closes #29639

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Clarifies the model and small_model config values in packages/web/src/content/docs/config.mdx. The docs now state that both values use provider_id/model_id, point users to opencode models and Models.dev for built-in provider/model IDs, explain how custom provider IDs map to the config keys, and note that small_model can use a different provider from the main model.

This addresses the confusion in #29639 about which part of provider/model is the provider and confirms the cross-provider small_model behavior.

How did you verify your code works?

• npm exec --yes prettier@3.6.2 -- --check packages/web/src/content/docs/config.mdx
• GitHub PR checks pass

I could not run the local Astro check because bun is not installed in this environment.

Screenshots / recordings

N/A, documentation-only change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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