Add migration guide for Persistent Query Settings

[radoslawszulgo]

Documented the procedure for migrating Persistent Query Settings between clusters, including export and import steps with examples.

[Copilot]

Pull Request Overview

This PR adds a new migration guide for Persistent Query Settings, providing step-by-step instructions for transferring query settings between MongoDB clusters to maintain consistent query performance during migrations.

• Added comprehensive migration documentation with export and import procedures
• Included practical examples showing both representativeQuery and queryShapeHash approaches
• Provided sample JSON output and MongoDB commands for real-world implementation

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[Copilot]

The JSON example contains new Date() which is JavaScript syntax, not valid JSON. In actual JSON output, this would be an ISODate string or timestamp object. Consider showing the actual JSON format that would be exported.

Suggested change

	"filter": { "signup_date": { "$gte": new Date("2024-01-01T00:00:00.000Z") } },
	"filter": { "signup_date": { "$gte": "2024-01-01T00:00:00.000Z" } },

[radoslawszulgo]

Examples and commands verified.

[nastena1606]

Why is the doc in reference? It's a usage instruction, not a reference material from my perspective. Upstream has it in reference too, but maybe they are wrong?