Add example of provisioning an azure virtual network

[interurban]

Community suggestion on an examples gap:

End-to-end example of provisioning an @azure virtual network w. network security groups/route table example:

https://github.com/search?q=repo%3Apulumi%2Fexamples%20VirtualNetwork&type=code

Pulumi AI: Pulumi AI helped, I had to be very specific though. The trick is to pass the Id of the provisioned route-table/nsg into a new Args instance on the vnet itself. It would probably help if official examples on Azure virtual networks were added to the repo.