Fix API Playground button opening not connected to instance on API Playground

When clicking on the API Playground link from the browser extension the API Playground page is not opening connected to the instance and is opening the github hosted version of API Playground.
see attachment

It would be better if:

• link goes to ` {NUXEO_SERVER}

  /nuxeo/playground` instead of https://nuxeo.github.io/api-playground/

• link only appears in the Useful links menu if the api-playground addon is installed on the Nuxeo instance