Add doc for the Websocket standard 3scale policy

The Websocket policy was added in 3scale 2.8 but it looks like we never documented it. This is the engineering Jira that created the Websocket policy:

https://issues.redhat.com/browse/THREESCALE-4051

This is a Jira for related documentation:

https://issues.redhat.com/browse/THREESCALE-4100 

The required doc is small and simple. The source for the policy is here:
https://github.com/3scale/APIcast/tree/master/gateway/src/apicast/policy/websocket 

Since no one has missed the doc for this policy, and since we reorganized policy content in 2.11, I'm going to make this update in 2.12 and 2.11 but will not plan to update earlier versions. Doing so would require a separate update and commits, but it does not seem worthy of the time to do it.