docs(provider): document custom registry format

[LouisDM]

Related Issue

Refs #1366

Problem

The provider documentation explains how to import a custom registry, but it does not document the api.json shape, required fields, capability metadata, or refresh behavior. Users therefore have to infer the format from implementation details.

What changed

• add a minimal, neutral custom-registry example in both English and Chinese
• document provider and model fields, supported provider protocols, and capability mapping
• document same-URL refresh behavior, including additions, removals, metadata updates, and preservation boundaries
• link the existing custom-registry entry point to the new section

This is intentionally limited to the documentation request. It does not add the longer-term /models discovery/import idea mentioned in the issue.

Verification

• pnpm -C docs run build
• git diff --check

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• Documentation-only change; verified with a full VitePress build.
• Ran gen-changesets; this documentation-only PR needs no changeset.
• Ran gen-docs and verified the generated documentation build.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 13bef00

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