[skip-ci] List count under Aggregation (#6711)
[skip-ci] List count under Aggregation (#6711)

It's currently listed under "Missing Value Handling" but is also an Aggregation
pydata/xarray,jhamman/xarray,jhamman/xarray,markelg/xray,pydata/xarray,pydata/xarray,markelg/xray,jhamman/xarray,markelg/xray