Update planner-overview.md to include reference to change notifications

[jennifershowe]

Added a section to let customers know they can use change notifications for sync scenarios.

Important

Required for API changes:

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[learn-build-service-prod]

Learn Build status updates of commit 7154c82:

✅ Validation status: passed

File	Status	Preview URL	Details
api-reference/v1.0/resources/planner-overview.md	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
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[learn-build-service-prod]

Learn Build status updates of commit 13d69c1:

✅ Validation status: passed

File	Status	Preview URL	Details
api-reference/v1.0/resources/planner-overview.md	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[JarbasHorst]

The https://learn.microsoft.com/en-us/graph/change-notifications-overview topic doesn't mention support for change notifications for Planner resources. How can an app subscribe to change notifications for a Planner resource?