docs: make universe

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

docs/reference/universe/os.md

@@ -0,0 +1,49 @@
+---
+sidebar_label: os
+---
+
+# dagger.io/os
+
+## #Container
+
+Built-in container implementation, using buildkit
+
+### #Container Inputs
+
+_No input._
+
+### #Container Outputs
+
+_No output._
+
+## #Dir
+
+### #Dir Inputs
+
+_No input._
+
+### #Dir Outputs
+
+_No output._
+
+## #File
+
+Built-in file implementation, using buildkit
+
+### #File Inputs
+
+_No input._
+
+### #File Outputs
+
+_No output._
+
+## #ReadDir
+
+### #ReadDir Inputs
+
+_No input._
+
+### #ReadDir Outputs
+
+_No output._