Review and update possible reference to nuxeo.io in documentation

The creation of nuxeo.io instance when people register to a trial will be disabled. We need to check and update the documentation so that it doesn't state that registering for a trial creates an instance on nuxeo.io.
Check Studio documentation (Connect part), Studio and IDE quick starts.

Leave the page describing nuxeo.io online because existing instances won't be disabled for the moment.