docs(migration): add docs on how to migrate from GH MQ

[jd]

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[Copilot]

Pull Request Overview

This PR adds documentation for migrating from GitHub Merge Queue to Mergify. The documentation provides a technical mapping between GitHub Merge Queue concepts and Mergify features, along with an incremental migration strategy.

• Adds comprehensive migration guide from GitHub Merge Queue to Mergify
• Updates navigation to include the new migration documentation
• Extends the main migration page with the new option

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/content/navItems.tsx	Adds navigation item for GitHub Merge Queue migration guide
src/content/docs/migrate/github-merge-queue.mdx	Creates new comprehensive migration documentation with concept mapping and migration strategy
src/content/docs/migrate.mdx	Updates main migration page to include GitHub Merge Queue option

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[mergify]

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