[Form] Added documentation for Form Events

Added initial documentation for Form Events:

* List of events
* Use cases for each event
* Description of the events workflow
* Flow charts for illustration

Updated PR with comments from @WouterJ and @MyKiwi

Took into account @cordoval's feedback

Additional improvements

* Added more examples from the Form component's code
* Moved the cookbook's 'register a subscriber using a callable' section into the form events documentation

Took into account @xabbuh's feedback

Fixed missing new line

Removed superfluous comma.

Updated from feedback

Moved the list of events down in the listener and subscriber section

* Formatted the list as a table, exposing the constant values;
* Added references to the form event information table;
* Added caution sections for actions that should prevented while in each event.

Added the form_events link in the components map file, a.k.a. 'Added the missing link'

Added missing newlines

Added cross-reference in the form events cookbook

Updated based on Luis' feedback

Added tables describing the form and event values:

* Model data
* Normalized data
* View data
* Event data

Small fixes

Updated a few wordings to make them more clear

Changed important directive to caution

Removed sidebar section about form extensions.

Renamed ``{EVENT}`` to ``FormEvents::{EVENT}``, and added code snippet

Last small changes

Fixed underline too short
feature #3704 [Form] Added documentation for Form Events (csarrazi)

This PR was merged into the 2.3 branch.

Discussion
----------

[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

Commits
-------

98de95a [Form] Added documentation for Form Events