Updating Compose documentation

- Split out commands into individual pages for maintainability
- Add full usage in commands pages for usability
- Updated description of the run command
- adding in Aanand's comment

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

docs/reference/start.md

@@ -0,0 +1,17 @@
+<!--[metadata]>
++++
+title = "start"
+description = "Starts existing containers for a service."
+keywords = ["fig, composition, compose, docker, orchestration, cli,  start"]
+[menu.main]
+parent = "smn_compose_cli"
++++
+<![end-metadata]-->
+
+# start
+
+```
+Usage: start [SERVICE...]
+```
+
+Starts existing containers for a service.