Update device plugin documentation + related pages (#14331)

* Update device plugin docs for Kubernetes 1.14

& related pages

This change is mainly about feature-state shortcodes & similar.

(if there's a way to get the short version string, eg "1.14" rather
than "v1.14", then that could go in place of the hard coded value
in this commit).

* Fix code block formatting

eg code blocks incorrectly marked as:
```shell

* Use glossary shortcodes where appropriate

* Hyperlink to Prometheus

* Tidy Markdown formatting

* Change example vendor domain name

Use a name inside ".example" to highlight that this is a DNS
domain name.

* Reword device plugins documentation

* Tweak headings for device plugins

* Add "what's next" to device plugins docs

* Tweak wording for device plugins docs

* Add KubeVirt device plugins

content/en/docs/tasks/configure-pod-container/extended-resource.md

@@ -6,10 +6,10 @@ weight: 40
 
 {{% capture overview %}}
 
-This page shows how to assign extended resources to a Container.
-
 {{< feature-state state="stable" >}}
 
+This page shows how to assign extended resources to a Container.
+
 {{% /capture %}}
 
 
@@ -141,9 +141,3 @@ kubectl delete pod extended-resource-demo-2
 * [Advertise Extended Resources for a Node](/docs/tasks/administer-cluster/extended-resource-node/)
 
 {{% /capture %}}
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-