Add FAQ entries for to_netcdf int64 error and ee.Geometry conversion

[jdbcode]

Adds two new FAQ entries to docs/faq.md:

1. ds.to_netcdf() fails with int64 cast error — Explains that Xee time coordinates use int64, which the scipy netCDF writer cannot handle (netCDF3 is limited to int32). Documents that Xarray selects backends in order netcdf4 → h5netcdf → scipy, so the error only occurs when neither netcdf4 nor h5netcdf is installed. Provides three fixes: install netCDF4, install h5netcdf, or pass engine= explicitly to .to_netcdf(). Also mentions .to_zarr() as an alternative. Relates to issue Writing NetCDF file fails with ValueError #194.

2. Shapely geometry conversion — Updates the existing shapely entry to include the concrete conversion snippet shapely.geometry.shape(ee_geom.getInfo()) for users passing an ee.Geometry to fit_geometry.