Docs: move advanced topic "create your own base image" out of basic user guide

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)

docs/sources/articles/index.rst

@@ -10,3 +10,4 @@ Contents:
    :maxdepth: 1
 
    security
+   baseimages

docs/sources/use/index.rst

@@ -14,7 +14,6 @@ Contents:
 
    basics
    workingwithrepository
-   baseimages
    port_redirection
    networking
    host_integration