Add col_pct_null() to API reference

Author: rich-iannone

docs/_quarto.yml

@@ -173,6 +173,7 @@ quartodoc:
         - name: Validate.rows_distinct
         - name: Validate.rows_complete
         - name: Validate.col_exists
+        - name: Validate.col_pct_null
         - name: Validate.col_schema_match
         - name: Validate.row_count_match
         - name: Validate.col_count_match