Docs: Update Inception provider documentation for mercury-2 model

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Summary

PR #12156 added the Inception Labs mercury-2 model with the following features:

• 128k context window
• Tool calling support
• OpenAI-compatible API

Current State

The Inception provider documentation at docs/customize/model-providers/top-level/inception.mdx exists but:

• Uses generic <MODEL_ID> placeholders instead of showing available models
• Does not mention mercury-2 or its capabilities (tool calling, 128k context)
• Does not document other models added (mercury-edit-2, mercury-coder-small)

Requested Changes

1. Add a section listing available Inception models:

   • mercury-2: 128k context, tool calling support
   • mercury-edit-2: Edit-optimized model
   • mercury-coder-small: Smaller coding model

2. Add example configuration specifically for mercury-2:

   models:
     - name: Mercury 2
       provider: inception
       model: mercury-2
       apiKey: ${{ secrets.INCEPTION_API_KEY }}

3. Mention tool calling capability for mercury-2 model

Related

• PR feat: add Inception Labs mercury-2 model #12156: feat: add Inception Labs mercury-2 model