Create documentation about trust boundaries of the .NET SDK and how to use it securely

[dsplaisted]

We should have documentation about the .NET SDK's trust boundary and how to use it securely. It should include:

Guidance:

• Don't build projects from untrusted sources
• Don't build projects from inside the Downloads folder
• Don't build projects where the parent folder hierarchy up to the file system root isn't trusted (because of things like Directory.Build.props)

Local trust boundary:

• SDK Installation
• Project sources and all parent folders up to the file system root
• Home folder
• Environment variables

Also trusted:

• NuGet packages
• NuGet sources

[Forgind]

<security>

[marcpopMSFT]

@dotnet/msbuild we should build a shared document for this to help customers understand the trust areas for their builds. We want to link to this documentation from #45341.