Add missing documentation for sln, buildprops, and buildtargets item templates

[Copilot]

This PR adds comprehensive documentation for three .NET SDK item templates that were missing from the official documentation:

buildprops template

• Creates a Directory.Build.props file for customizing MSBuild properties across an entire folder tree
• Documents the --inherit option for importing parent Directory.Build.props files
• Documents the --use-artifacts option for enabling artifacts output layout with reference to artifacts documentation
• Added in .NET SDK 8.0.100

buildtargets template

• Creates a Directory.Build.targets file for customizing MSBuild targets and tasks across an entire folder tree
• Documents the --inherit option for importing parent Directory.Build.targets files
• Added in .NET SDK 8.0.100

sln template

• Creates an empty solution file containing no projects
• Documents the --format option available in .NET SDK 9.0.200+ for choosing between sln and slnx formats
• Notes that .NET 10 will default to slnx format

Documentation Updates

• Added the new templates to the templates table of contents in includes/templates.md with proper cross-references
• Enhanced the --use-artifacts option description with a link to the artifacts output layout documentation
• All sections follow existing documentation patterns with clear descriptions and detailed option explanations

All template options have been tested and verified for accuracy using the actual .NET SDK commands.

Fixes #48190.

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Internal previews

📄 File	🔗 Preview link
docs/core/tools/dotnet-new-sdk-templates.md	Default templates for dotnet new

[baronfel]

@copilot the new templates need to be added to or linked from the existing includes/templates.md table of contents

[Copilot]

@copilot the new templates need to be added to or linked from the existing includes/templates.md table of contents

Added the buildprops and buildtargets templates to the templates.md table of contents with proper links to their documentation sections. The sln template was already listed but now has the correct link. Changes committed in 4891e45.