Add documentation for populating relations with GraphQL API

[dev-docs-github-app-dev]

This PR adds new documentation to explain how to use the populate parameter in the Strapi GraphQL API to fetch related data along with your main query.

The new documentation covers:

• Basic usage of the populate parameter to include related fields in GraphQL queries
• Deep population of nested relations using dot notation
• Populating media fields
• Populating dynamic zones with fragments

These updates will help developers better understand how to efficiently retrieve related data in a single GraphQL request, improving the performance and user experience of their Strapi-powered applications.

[dev-docs-github-app-dev]

This pull request was created by AI Agent. Please review the changes and provide feedback. Context used:

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