Move includes only used once into their articles

[adegeo]

Summary

• Deleted the includes and moved the content to the article.

Fixes #50697

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Internal previews

📄 File	🔗 Preview link
docs/standard/datetime/how-to-use-dateonly-timeonly.md	How to use the DateOnly and TimeOnly structures
docs/standard/datetime/system-text-json-support.md	DateTime and DateTimeOffset support in System.Text.Json

[Copilot]

Pull request overview

This PR eliminates unnecessary include files by inlining content that was only referenced once. The linked issue (#50697) identified two include files (dateonly-and-timeonly-serialization.md and dateonly-and-timeonly-serialization-cs.md) that were each used in only one location, making them candidates for removal.

Changes:

• Deleted two single-use include files in the docs/standard/datetime/includes/ directory
• Inlined the DateOnly and TimeOnly serialization content into the two articles that referenced them
• Added HTML comments noting that content is duplicated between the two articles

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
docs/standard/datetime/system-text-json-support.md	Replaced include reference with inline content (C# only)
docs/standard/datetime/how-to-use-dateonly-timeonly.md	Replaced include reference with inline content (C# and VB)
docs/standard/datetime/includes/dateonly-and-timeonly-serialization.md	Deleted (was only used once)
docs/standard/datetime/includes/dateonly-and-timeonly-serialization-cs.md	Deleted (was only used once)

[BillWagner]

This LGTM @adegeo

Let's