Add mssubservice to docfx.json

[mattwojo]

Fixes #

Docs for Toolkit PR #

What changes to the docs does this PR provide?

PR Checklist

Please check if your PR fulfills the following requirements:

• Correctly picked the right branch to base the change off (dev for new features, main for typos/improvements)
• For new pages, used the provided template
• For new features, added an entry in the Table of Contents
• Ran against a spell and grammar checker
• Contains NO breaking changes

Other information

[learn-build-service-prod]

Learn Build status updates of commit 7c9a8a7:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/docfx.json	✅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 6bd1ed0:

💡 Validation status: suggestions

File	Status	Preview URL	Details
dotnet/docfx.json	💡Suggestion		Details
docs/docfx.json	✅Succeeded

dotnet/docfx.json

• Line 1, Column 1: [Suggestion: uhf-header-id-missing - See documentation] uhfHeaderId should not be null; specify the uhfHeaderId in global metadata of docfx.json file.

For more details, please refer to the build report.

Note: Your PR may contain errors or warnings or suggestions unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.

For any questions, please:

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