docs: add stubs for all commands

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/buildx_du.md

@@ -0,0 +1,16 @@
+# buildx du
+
+```
+Usage:  docker buildx du
+
+Disk usage
+
+Options:
+      --builder string   Override the configured builder instance
+      --filter filter    Provide filter values
+      --verbose          Provide a more verbose output
+```
+
+## Description
+
+Disk usage