DOC: Add datatest package to list of third-party extension accessors.
This change adds the `datatest` package to the list of third-party extension accessors. Since `datatest` supports more than `pandas`, I wasn't sure if it was appropriate to add it to the **Data cleaning and validation** section, so I only added it to the accessors list for now.

Unfortunately, the "Classes" column was not wide enough--so this patch affects all rows in the table. This shouldn't happen again as the column is now wide enough to accommodate all of the classes that support extension accessors.