Add a Cloudfront section to the amazon s3 documentation

There documentation at https://doc.nuxeo.com/nxdoc/amazon-s3-online-storage/ does not have a section about cloudfront configuration.

The README at https://github.com/nuxeo/nuxeo-core-binarymanager-cloud/tree/master/nuxeo-core-binarymanager-s3 contains the info. However it assumes advanced knowledge of cloudfront

Some missing part are:

• enabling Query String Forwarding and Caching in the aws console for the cloudfront distribution (https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/distribution-web-values-specify.html#DownloadDistValuesQueryString)
• nuxeo.s3storage.directdownload=true must be set in nuxeo.conf