Update usage examples to include dotnet tool execute (dnx)

[jeffkl]

Is your feature request related to a problem? Please describe.
.NET 10 SDK shipped with new functionality, dotnet tool execute or dnx for short. Its useful for executing .NET tools without having to install them first.

Describe the solution you'd like
Usage for Sign CLI should include something like

dotnet tool execute sign --prerelease code -?

or just

dnx sign --prerelease -?

This is handy for hosted build environments because it simplifies the execution by not having to install the tool.

    # Run the signing command
    - pwsh: |
        dnx sign --prerelease code azure-key-vault `
        "**/*.nupkg" `
        --base-directory "$(Pipeline.Workspace)\BuildPackages" `
        --file-list "$(Pipeline.Workspace)\config\filelist.txt" `
        --publisher-name "Contoso" `
        --description "One Sign CLI demo" `
        --description-url "https://github.com/dotnet/sign" `
        --azure-key-vault-tenant-id "$(SignTenantId)" `
        --azure-key-vault-client-id "$(SignClientId)" `
        --azure-key-vault-client-secret '$(SignClientSecret)' `
        --azure-key-vault-certificate "$(SignKeyVaultCertificate)" `
        --azure-key-vault-url "$(SignKeyVaultUrl)"
      displayName: Sign packages

Describe alternatives you've considered

Additional context
Include the output of sign --version.
Add any other context about the problem here.