Add AWS Bedrock Mantle (OpenAI Compatible endpoints) example to conversational search documentation

[tbuatois]

Pull Request

Related issue

Fixes #<issue_number>

What does this PR do?

• Adds AWS Bedrock Mantle integration documentation to the conversational search guide
• Includes configuration example showing how to use Bedrock Mantle with the OpenAI-compatible source
• Documents the baseUrl parameter requirement for Bedrock Mantle
• Adds important note about API key region requirements (API keys must be generated in the same region as the endpoint)
• Provides example model ID (openai.gpt-oss-120b) and endpoint format
• Updates the baseUrl field description to include AWS Bedrock Mantle as a required source

Dependencies

This documentation requires Meilisearch PR meilisearch/meilisearch#5905 to be merged, which fixes the baseUrl parameter handling for OpenAI-compatible sources like Bedrock Mantle.

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• Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
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