[GroupPolicy docs] Add new "Default Terminal Application" policy

[htcfreek]

Adds documentation for the new policy to configure the default Terminal application.

[learn-build-service-prod]

Learn Build status updates of commit a0880bc:

✅ Validation status: passed

File	Status	Preview URL	Details
TerminalDocs/group-policy.md	✅Succeeded	View

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[htcfreek]

@DHowett , @mattwojo ,@nguyen-dows
Thia PR should merge now. The new admx are released.

[learn-build-service-prod]

Learn Build status updates of commit e43afd4:

✅ Validation status: passed

File	Status	Preview URL	Details
TerminalDocs/group-policy.md	✅Succeeded	View

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• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit 0b67537:

✅ Validation status: passed

File	Status	Preview URL	Details
TerminalDocs/group-policy.md	✅Succeeded	View

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

[DHowett]

Thanks!