Use Generator instead of Nuxeo IDE in How-tos

In how-tos, we should now provide steps that use the generator instead of Nuxeo IDE. Pages to update are:

• https://doc.nuxeo.com/nxdoc/how-to-contribute-to-an-extension/
• https://doc.nuxeo.com/nxdoc/how-to-add-field-validation/
• https://doc.nuxeo.com/nxdoc/start-customizing-the-nuxeo-platform/
  (see NXDOC_959 for page getting-started-with-nuxeo-ide/ )
• https://doc.nuxeo.com/nxdoc/how-to-implement-local-groups-or-roles-using-computed-groups/