Add warning for untrusted .NET templates

[marcpopMSFT]

Added a warning about untrusted .NET templates.

Summary

We wanted to make sure customers knew that installing and running untrusted templates came with risk. Let me know if I should update the dotnet new install documentation as well.

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Internal previews

📄 File	🔗 Preview link
docs/core/tools/custom-templates.md	docs/core/tools/custom-templates
docs/core/tools/dotnet-new-install.md	docs/core/tools/dotnet-new-install

[Copilot]

Pull Request Overview

This PR adds a security warning to inform users about the risks of installing and running untrusted .NET templates. The warning explains that templates can execute MSBuild code, making them potentially dangerous if they come from untrusted sources.

• Added a warning callout about security risks of untrusted templates

[meaghanlewis]

@marcpopMSFT thanks for this PR! I left a warning style and wording change update for you.

And yes, please feel free to update the dotnet new install documentation too.

[blowdart]

I would be more formal than @meaghanlewis suggested with

Templates can run MSBuild code when triggered, do not install or run untrusted .NET templates.

(italics to highlight my wording change, not for the final edit)

[marcpopMSFT]

Thank you both. I used blowdart text. I also added it to the dotnet new install documentation as well. I put it up near the top where it's less likely to be missed. I thought about putting it at the bottom or the end of the description section but was worried it would blend in too much. Thoughts on where I put it on that doc, @meaghanlewis ?

[meaghanlewis]

@marcpopMSFT thanks for the updates!