add REST documentation for content module

Actually, it returns a JSON object so we'll still need a model

````
{
    'url': "/etherpad/0/oae/g.t0Ksj73idCX2TbcE$c_Tenant_ZytOGdxrb?sessionID=s.4fxYZbycjVTSqymd&language=en&pathPrefix=%2Fetherpad%2F0"
}
```