Add performance and pagination guidance to FhirEngine.search KDoc (#2684)

[himanshu-pandey-git]

Summary

This PR improves the documentation for the FhirEngine.search() function by adding performance tips and usage recommendations. It guides developers to avoid fetching large datasets in a single call and instead use .count() and .from() for paginated queries.

Changes

• Merged the basic usage example and performance best practices into the KDoc for FhirEngine.search.
• Linked to relevant issue and metrics PR for context.

References

• Closes #2684
• Related PR: https://github.com/google/android-fhir/pull/2669

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