Add an example for using the compound search operator

[GromNaN]

Q	A
Type	doc
BC Break	no
Fixed issues	-

Summary

The compound search operator is very common for combining multiple search operators, its usage is more complex, that's why it needs a dedicated example.

[Copilot]

Pull Request Overview

This PR adds documentation for the compound search operator in MongoDB Atlas Search, demonstrating how to combine multiple search operators. The compound operator is a commonly used feature that requires clearer documentation due to its complexity.

Key changes:

• Added example code showing usage of compound(), must(), and should() operators
• Included explanation text linking to official MongoDB documentation

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