docs: Add types reference

Signed-off-by: Helder Correia <174525+helderco@users.noreply.github.com>

pkg/dagger.io/dagger/types.cue

@@ -10,11 +10,6 @@ package dagger
 	$dagger: fs: _id: string | null
 }
 
-// An empty directory
-#Scratch: #FS & {
-	$dagger: fs: _id: null
-}
-
 // A reference to an external secret, for example:
 //  - A password
 //  - A SSH private key

pkg/dagger.io/dagger/values.cue

@@ -0,0 +1,6 @@
+package dagger
+
+// An empty filesystem tree
+#Scratch: #FS & {
+	$dagger: fs: _id: null
+}