Update Reverse Proxy and Authentification documentations

We had some feedbacks on the following pages which need to be updated:

• https://doc.nuxeo.com/display/NXDOC/Authentication+and+User+Management (FT and LTS2015 at least)
• https://doc.nuxeo.com/60/admindoc/configuring-a-reverse-proxy-to-work-with-live-edit-and-client-certificate-authentication/: Page was removed after 6.0 after Drive replaced Live Edit. Maybe create a page again in recent versions with content about Certificate Authentication that is generic and not specfic to Live Edit.

It seems the customer finds the pages from 5.x are more complete than the current ones. (EDIT: links updated with pages in more recent versions).