Add alert banner to higlight the decommissioning of the SMS. (#80)

Update Streaming Media Service (SMS) page with an alert banner to highlight
that the service is being decommissioned.

docs/services/streaming-media.md

@@ -3,6 +3,9 @@
 This page gives an overview of the Streaming Media Service, describing its current status, where and
 how it's developed and deployed, and who is responsible for maintaining it.
 
+!!! warning
+    This service is being decommissioned. Please use alternative services for publishing new content (see the [SMS closure news article](https://help.uis.cam.ac.uk/news/sms-future-feb2021) for more information).
+
 ## Service Description
 The Streaming Media Service (SMS) provides a video and audio transcoding and hosting service.