Update usage examples to include dotnet tool execute (dnx)

**Is your feature request related to a problem? Please describe.**
.NET 10 SDK shipped with [new functionality](https://learn.microsoft.com/dotnet/core/whats-new/dotnet-10/sdk#one-shot-tool-execution), `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.

```yaml
# 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.