[READY][3.8.4] Add API webhooks doc

Refers to https://github.com/alchemy-fr/Phraseanet/pull/1033

**Remaining tasks**

- [x] Add english version
- [x] Data format for webhooks is not yet determined
A *webhook* can be configured to trigger one or more events.
The client application must support the listening to these events.

.. note::

    Phraseanet trigger *webhooks* only when a new entry is created in a 
    RSS feed.
    For instant, It is not possible to configure the type of event to trigger.
    But we stand prepared for this possibility in the near future.