Add gitignore template to dotnet new SDK templates documentation

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Summary

The dotnet new gitignore template was missing from the SDK templates documentation table despite being a valid, preinstalled template since .NET SDK 3.0.

Added gitignore template entry to the Preinstalled templates table:

• Template: .gitignore file
• Short name: gitignore
• Tags: Config
• Introduced: 3.0
• Template options: None

Placed alphabetically among Config templates, between EditorConfig and global.json. Formatted consistently with other option-less templates (plain text short name without brackets/link).

Original prompt

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This section details on the original issue you should resolve

<issue_title>[New article] Configuring a .NET project for Git</issue_title>
<issue_description>### Describe the issue or suggestion

There's no documentation for the dotnet new gitignore command or other setup related to configuring .NET projects for Git integration.

It looks like this was originally addressed in #18262, but dotnet new <TEMPLATE> page has since removed any mention of dotnet new gitignore.

At a minimum, we could add a remark similar to what we have for global.json on that docs page.</issue_description>

<agent_instructions>Add a row to the table with the details for gitignore: https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-new-sdk-templates#preinstalled-templates. The Template is: gitignore, the Short name is: gitignore, the Tag is: Config, and this was Introduced in: 3.0. There are no template options to link to from the short name.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@meaghanlewis @MackinnonBuck I'm going to add details about `dotnet new gitignore` to: https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-new-sdk-templates#preinstalled-templates

• Fixes [New article] Configuring a .NET project for Git #50389

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Internal previews

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

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Pull request overview

This PR adds the missing gitignore template to the SDK templates documentation table. The template has been available since .NET SDK 3.0 but was previously undocumented in this reference table.

• Added a new row for the .gitignore file template in the Preinstalled templates table
• Correctly positioned alphabetically between EditorConfig and global.json
• Followed the pattern for templates without template-specific options (no link/brackets on short name)