Add explanation about fill value meaning across software library and standards

[maxrjones]

This is a common difficulty when working with datacubes across formats.
Background information:

• Don't write empty chunks to icechunk zarr-developers/VirtualiZarr#745 (comment)
• netCDF this is the historical meaning of _FillValue: docs.unidata.ucar.edu/nug/2.0-draft/nug_conventions.html#FillValue.
• overview from Ryan - Is _FillValue really the same as zarr's fill_value? pydata/xarray#5475 (comment)