XIOS usage need update and relocation

During the gitlab spring cleaning, it emerged that the description of XIOS need some update and, more interestingly, its usage description is located in the advanced user section and a bit "buried" within the diagnostic section.

As XIOS is a core component of NEMO I/O, its description should be part of the basics chapter with an explicit title to help users getting info about how to use it.

I suggest to move the sections Extracting and installing XIOS and Modes within Download and install the NEMO code within a new section named Install XIOS library and replace Diagnostics with Add new diagnostics.