Add namespace summaries for System* namespaces

[gewarren]

• Add missing summaries
• Delete ns* files for nonexistent namespaces

Fixes #10998

[learn-build-service-prod]

Learn Build status updates of commit b7d58ed:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/ns-System.Cloud.DocumentDb.xml	✅Succeeded	View
xml/ns-System.Cloud.Messaging.xml	✅Succeeded	View
xml/ns-System.Collections.Frozen.xml	✅Succeeded	View
xml/ns-System.CommandLine.Binding.xml	✅Succeeded	View
xml/ns-System.CommandLine.Builder.xml	✅Succeeded	View
xml/ns-System.CommandLine.Completions.xml	✅Succeeded	View
xml/ns-System.CommandLine.Help.xml	✅Succeeded	View
xml/ns-System.CommandLine.Invocation.xml	✅Succeeded	View
xml/ns-System.CommandLine.IO.xml	✅Succeeded	View
xml/ns-System.CommandLine.Parsing.xml	✅Succeeded	View
xml/ns-System.CommandLine.xml	✅Succeeded	View
xml/ns-System.Data.Sql.xml	✅Succeeded	View
xml/ns-System.Drawing.Design.xml	✅Succeeded	View
xml/ns-System.Formats.Nrbf.xml	✅Succeeded	View
xml/ns-System.Globalization.xml	✅Succeeded	View
xml/ns-System.Net.Http.xml	✅Succeeded	View
xml/ns-System.Net.ServerSentEvents.xml	✅Succeeded	View
xml/ns-System.Numerics.Tensors.xml	✅Succeeded	View
xml/ns-System.Runtime.Diagnostics.xml	✅Succeeded	n/a (file deleted or renamed)
xml/ns-System.Runtime.Intrinsics.Arm.Arm64.xml	✅Succeeded	n/a (file deleted or renamed)
xml/ns-System.Runtime.Intrinsics.Arm.xml	✅Succeeded	View
xml/ns-System.Runtime.Loader.xml	✅Succeeded	View
xml/ns-System.Runtime.Remoting.Channels.Smtp.xml	✅Succeeded	n/a (file deleted or renamed)
xml/ns-System.Runtime.xml	✅Succeeded	View
xml/ns-System.Text.Json.Schema.xml	✅Succeeded	View

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For more details, please refer to the build report.

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[BillWagner]

This LGTM @gewarren