docs: split reference docs to separate files

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

docs/reference/buildx_ls.md

@@ -0,0 +1,21 @@
+# `buildx ls`
+
+## Description
+
+Lists all builder instances and the nodes for each instance
+
+Example:
+
+```console
+$ docker buildx ls
+
+NAME/NODE       DRIVER/ENDPOINT             STATUS  PLATFORMS
+elated_tesla *  docker-container
+  elated_tesla0 unix:///var/run/docker.sock running linux/amd64
+  elated_tesla1 ssh://ubuntu@1.2.3.4        running linux/arm64, linux/arm/v7, linux/arm/v6
+default         docker
+  default       default                     running linux/amd64
+```
+
+Each builder has one or more nodes associated with it. The current builder's
+name is marked with a `*`.