Update dotnet nuget why docs with assets file info

[zivkan]

When this is complete:

• Enable dotnet nuget why on non-SDK style projects #13576

We need to update the docs for dotnet nuget why so that we can explain that non-sdk projects can restore with msbuild -t:restore, then use msbuild path\to\project.csproj -getProperty:ProjectAssetsFile, and then pass that assets file path to dotnet nuget why.