Nginx doc

This PR review OMERO.web deployment for various purposes. Basically it split instructions for developers and system administrators to not confuse each other.

My only comment is that it is nasty that ```static_url``` doesn't pick up ```force_script_name``` but I have no way of knowing what was set in previous deployment. Either we leave it as it is or doing it in a right way we will need to add instruction to upgrade script as it will break existing deployment scripts
This will append the content if `etc/nginx.conf` already exist. Use `>` instead?