Splitting out the cli command into parts.

Signed-off-by: Mary Anthony <mary@docker.com>

docs/reference/commandline/push.md

@@ -0,0 +1,19 @@
+<!--[metadata]>
++++
+title = "push"
+description = "The push command description and usage"
+keywords = ["share, push, image"]
+[menu.main]
+parent = "smn_cli"
+weight=1
++++
+<![end-metadata]-->
+
+# push
+
+    Usage: docker push NAME[:TAG]
+
+    Push an image or a repository to the registry
+
+Use `docker push` to share your images to the [Docker Hub](https://hub.docker.com)
+registry or to a self-hosted one.