Added global metadata for ms.topic as fallback

[kcpitt]

When ms.topic values isn't present within a file, the default value will be "reference" to assist with reporting.

[learn-build-service-prod[bot]]

Learn Build status updates of commit 1559b6c:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
docfx.json	:white_check_mark:Succeeded

For more details, please refer to the build report.

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