Move pages under tldr, lint run.sh, update docs

tldr/u3d

@@ -0,0 +1,29 @@
+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# u3d
+
+> A set of tools to interact with Unity.
+> More information: <https://github.com/DragonBox/u3d>.
+
+- Open the project in the current directory using the 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}}`