Add XIOS3 demonstrator section

v5 will include example usage of some of the new features available with XIOS3 in a modified reference configuration (X3_ORCA2_ICE_PISCES). A new section is added to the user guide to explain the changes that have been made to the XML files to support these features.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information