[excel] (Licensing) Add step to join Insiders for personal scripts

[AlexJerabek]

While Scripts for home and family are in preview, users will need to join the M365 Insider program.

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

Learn Build status updates of commit eae609e:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
docs/includes/platform-requirements.md	:white_check_mark:Succeeded	View

For more details, please refer to the build report.

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