docs: ♻️ merge API Reference and Universe menu entries

Signed-off-by: jffarge <jf@dagger.io>

docs/reference/os.md

@@ -0,0 +1,55 @@
+---
+sidebar_label: os
+---
+
+# alpha.dagger.io/os
+
+OS operations
+
+```cue
+import "alpha.dagger.io/os"
+```
+
+## os.#Container
+
+Built-in container implementation, using buildkit
+
+### os.#Container Inputs
+
+_No input._
+
+### os.#Container Outputs
+
+_No output._
+
+## os.#Dir
+
+### os.#Dir Inputs
+
+_No input._
+
+### os.#Dir Outputs
+
+_No output._
+
+## os.#File
+
+Built-in file implementation, using buildkit A single file
+
+### os.#File Inputs
+
+_No input._
+
+### os.#File Outputs
+
+_No output._
+
+## os.#ReadDir
+
+### os.#ReadDir Inputs
+
+_No input._
+
+### os.#ReadDir Outputs
+
+_No output._