u3d: add page (#7260)

pages/common/u3d.md

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+# u3d
+
+> Set of tools to interact with Unity from command line.
+> More information: <https://github.com/DragonBox/u3d>.
+
+- Open project from the current directory in correct Unity version:
+
+`u3d`
+
+- List installed versions of Unity:
+
+`u3d list`
+
+- List available versions of Unity that can be downloaded:
+
+`u3d available`
+
+- Download and install latest stable Unity version:
+
+`u3d install latest_stable`
+
+- Download and install Unity version and editor [p]ackages:
+
+`u3d install {{2021.2.0f1}} -p {{Unity,iOS,Android}}`