MOM6: Moved to flat (no subdirs) mode for doxygen

- NOAA-GFDL/MOM6@90d31ed Moved to flat (no subdirs) mode for doxygen
- NOAA-GFDL/MOM6@fd533dc Added draft of Diagnostics page
- NOAA-GFDL/MOM6@5bd2cfc Implemented xy-averaged diagnostics for interface data
- NOAA-GFDL/MOM6@ff858c7 Corrected variable names (changed extrinsic to extensive)
- NOAA-GFDL/MOM6@3aa0e78 Fix "edges" attribute for dimension variable in z-diag output
- NOAA-GFDL/MOM6@c32946c Fix subroutine name in error string. mom-ocean#334
- NOAA-GFDL/MOM6@58b26a0 Remove support for diagnostic horizontal averaging on Z interfaces. mom-ocean#334