Add response table to merge trains API

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What does this MR do?

Adds a response table for POST /projects/:id/merge_trains/merge_requests/:merge_request_iid to the merge trains API doc page.

Why?

To align with our style guidance for REST APIs.

Screenshots (if applicable)

How to setup/test

• Run gdk start and navigate to /help/api/merge_trains.
• Verify table renders and code blocks are unbolded.