Allow custom documenters in autosummary

Is your feature request related to a problem? Please describe.

I'd like to have autosummary be able to recognize autodoc documenters other than the built-in ones. E.g. celery tasks. Celery brings it's own documenter [1] in an extension, but the typ of task is not recognized.

[1] http://docs.celeryproject.org/en/latest/_modules/celery/contrib/sphinx.html#TaskDocumenter

Describe the solution you'd like

The code below could be turned into a loop with configurable inputs:

https://github.com/sphinx-doc/sphinx/blob/d501bc136ce5069e97a5f347dbc694717dddced8/sphinx/ext/autosummary/generate.py#L192-L198

The list could be automatically, or manually, extended with further documenters, via conf.py or from other extensions.

Describe alternatives you've considered

• configure via conf.py
• configure from other extension
• determine directly from registered documenters

(The last would be great - although a question is how to know what is available when adapting the jinja template)

Start in sphinx/ext/autosummary/generate.py around lines 192-198, where the built-in documenter types are handled. Read how autodoc documenters are registered and how conf.py or extensions could provide them, then check the Jinja template inputs. Done means autosummary recognizes registered custom documenters such as Celery's task documenter without breaking existing built-ins.