Add Groupby and Rolling methods to docs

[jhamman]

The injected apply/reduce methods for the Groupby and Rolling objects are not shown in the api documentation page. While there is obviously a fair bit of overlap between the similar DataArray/Dataset methods, it would help users to know what methods are available to the Groupby and Rolling methods if we explicitly listed them in the documentation. Suggestions on the best format to show these mehtods (e.g. Rolling.mean) are welcomed.

[keewis]

with #3602, the methods of any object can be found (and clicked on) on the object's API page (reachable from the main API page, but separate). This might not be ideal, but I'd say finding a better solution is out of scope for #3602.

[dcherian]

Fixed by #5903