Fold concept overview pages into section indexes

Where an existing Concept section has an index and the first page is
“Overview of X”, move the content of that overview into the index page
itself.

Also, configure redirects accordingly.

content/en/docs/concepts/containers/_index.md

@@ -1,5 +1,5 @@
 ---
-title: "Containers"
+title: Containers
 weight: 40
 description: Technology for packaging an application along with its runtime dependencies.
 reviewers:
@@ -8,3 +8,37 @@ reviewers:
 content_type: concept
 ---
 
+<!-- overview -->
+
+Each container that you run is repeatable; the standardization from having
+dependencies included means that you get the same behavior wherever you
+run it.
+
+Containers decouple applications from underlying host infrastructure.
+This makes deployment easier in different cloud or OS environments.
+
+
+
+
+<!-- body -->
+
+## Container images
+A [container image](/docs/concepts/containers/images/) is a ready-to-run
+software package, containing everything needed to run an application:
+the code and any runtime it requires, application and system libraries,
+and default values for any essential settings.
+
+By design, a container is immutable: you cannot change the code of a
+container that is already running. If you have a containerized application
+and want to make changes, you need to build a new container that includes
+the change, then recreate the container to start from the updated image.
+
+## Container runtimes
+
+{{< glossary_definition term_id="container-runtime" length="all" >}}
+
+## {{% heading "whatsnext" %}}
+
+* Read about [container images](/docs/concepts/containers/images/)
+* Read about [Pods](/docs/concepts/workloads/pods/)
+