[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
[80 signes et caractères par ligne]

Each OAuth2 client application can receive information from Phraseanet through 
*Webhooks*. Its purpose is to send HTTP requests with the POST verb to a given 
URL when an event occurs in Phraseanet.
These *Webhooks* can be used to perform specific actions in client application 
such as sending emails, running updates, notifiying of new flows...*etc*. 
Webhooks extend functionalities of the client application.