docs: add stubs for all commands

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

docs/reference/buildx.md

@@ -0,0 +1,32 @@
+# buildx
+
+```
+Usage:  docker buildx [OPTIONS] COMMAND
+
+Build with BuildKit
+
+Options:
+      --builder string   Override the configured builder instance
+
+Management Commands:
+  imagetools  Commands to work on images in registry
+
+Commands:
+  bake        Build from a file
+  build       Start a build
+  create      Create a new builder instance
+  du          Disk usage
+  inspect     Inspect current builder instance
+  ls          List builder instances
+  prune       Remove build cache
+  rm          Remove a builder instance
+  stop        Stop builder instance
+  use         Set the current builder instance
+  version     Show buildx version information
+
+Run 'docker buildx COMMAND --help' for more information on a command.
+```
+
+## Description
+
+Build with BuildKit