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

It's currently listed under "Missing Value Handling" but is also an Aggregation

Author: dcherian

doc/api.rst

@@ -200,6 +200,7 @@ Aggregation
    Dataset.any
    Dataset.argmax
    Dataset.argmin
+   Dataset.count
    Dataset.idxmax
    Dataset.idxmin
    Dataset.max
@@ -406,6 +407,7 @@ Aggregation
    DataArray.any
    DataArray.argmax
    DataArray.argmin
+   DataArray.count
    DataArray.idxmax
    DataArray.idxmin
    DataArray.max