Add docs for depend-defaults

[autoantwort]

For microsoft/vcpkg-tool#538

[prmerger-automator]

@autoantwort : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit 4d44a4d:

✅ Validation status: passed

File	Status	Preview URL	Details
vcpkg/reference/vcpkg-json.md	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

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[learn-build-service-prod]

Learn Build status updates of commit 06f299b:

✅ Validation status: passed

File	Status	Preview URL	Details
vcpkg/reference/vcpkg-json.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

[vicroms]

Drafting until the related features is merged and released.