[Form] Added documentation for Form Events

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | >=2.3
| Fixed tickets | Dunno

Added initial documentation for Form Events:

* List of events
* Use cases for each event
* Description of the events workflow
* Flow charts for illustration
* Registering event listeners or subscribers in a form
* Describing the data available for each event