DOC: Document grid_mapping in encoding

[stefank0]

I think there should be some more documentation about writing the grid_mapping to the encoding, for example to avoid the following issue: ds.to_netcdf(path, encoding=encoding) overwrites the grid_mapping encoding and results in incomplete CRS information (results in a NetCDF for which the CRS is not automatically recognized by QGIS, for example). Probably this is commonly used to set compression (accepted answer to https://stackoverflow.com/questions/40766037/specify-encoding-compression-for-many-variables-in-xarray-dataset-when-write-to).

• [ ] Closes #xxxx
• [ ] Tests added
• [ ] Fully documented, including docs/history.rst for all changes and docs/rioxarray.rst for new API

[snowman2]

Related: https://github.com/pydata/xarray/discussions/5709

[snowman2]

Thanks @stefank0 :+1: