Add documentation about Expressroute circuit, provisioning

ALZ accelerator, it looks like it will only provision the gateway, not the circuit, it would be great for some further information on the express route and adding the circuit, ie considerations around provisioned circuit etc

Start by reviewing the ALZ accelerator documentation for ExpressRoute and the gateway provisioning flow. Document whether the accelerator provisions only the gateway, how to add or provision the circuit, and relevant circuit-provisioning considerations. Done means the documentation clearly explains the distinction and next steps.

Written by the indexing model from the issue text.