Add resource for unity (#6639)

src/data/roadmaps/game-developer/content/unity-3d@rNeOti8DDyWTMP9FB9kJ_.md

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 # Unity 3D
 
 **Unity 3D** is a versatile, cross-platform game engine that supports the development of both 2D and 3D games. This game engine allows users to create a wide variety of games including AR, VR, Mobile, Consoles, and Computers in C#. It provides a host of powerful features and tools, such as scripting, asset bundling, scene building, and simulation, to assist developers in creating interactive content. Unity 3D also boasts a large, active community that regularly contributes tutorials, scripts, assets, and more, making it a robust platform for all levels of game developers.
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+- [@video@Unity in 100 Seconds](https://www.youtube.com/watch?v=iqlH4okiQqg)