Add explaination how to create virtual devices

While all features explained on this page are accessible from the device page, this is the only feature that has to be accessed from somewhere else, which might not be clear to new users.

docs/core-functionality/devices.md

@@ -118,3 +118,5 @@ Inventory items represent hardware components installed within a device, such as
 A virtual chassis represents a set of devices which share a single control plane: a stack of switches which are managed as a single device, for example. Each device in the virtual chassis is assigned a position and (optionally) a priority. Exactly one device is designated the virtual chassis master: This device will typically be assigned a name, secrets, services, and other attributes related to its management.
 
 It's important to recognize the distinction between a virtual chassis and a chassis-based device. For instance, a virtual chassis is not used to model a chassis switch with removable line cards such as the Juniper EX9208, as its line cards are _not_ physically separate devices capable of operating independently.
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+To add a virtual chassis, select two or more devices in the devices list, and click "Create Virtual Chassis" at the bottom of the page.