Add definition for Constrained Device Edge

[mtrifiro]

a subcategory of the User Edge consisting of microcontroller-based devices that are highly resource-constrained and distributed in the physical world. These devices range from simple, fixed-function sensors and actuators that perform very little to no localized compute to more capable devices such as Programmable Logic Controllers (PLCs), Remote Terminal Units (RTUs) and Engine Control Units (ECUs) addressing time- and safety-critical applications. Devices at this tier leverage embedded software and have the most unique form factors.