[docs] Add description for all the data grid events

[oliviertassinari]

• I have searched the issues of this repository and believe that this is not a duplicate.

Summary 💡

Add description for all the events. It's currently far from clear when each event is supposed to fire.

Examples 🌈

https://developer.mozilla.org/en-US/docs/Web/API/Element#events

I assume we could start with:

diff --git a/packages/grid/_modules_/grid/constants/eventsConstants.ts b/packages/grid/_modules_/grid/constants/eventsConstants.ts
index 74b08928..6d757731 100644
--- a/packages/grid/_modules_/grid/constants/eventsConstants.ts
+++ b/packages/grid/_modules_/grid/constants/eventsConstants.ts
@@ -27,7 +27,12 @@ export const GRID_CELL_VALUE_CHANGE = 'cellValueChange';

 export const GRID_CELL_EDIT_ENTER = 'cellEditEnter';
 export const GRID_CELL_EDIT_EXIT = 'cellEditExit';
+
+/**
+ * Fired when a keydown event has triggerd a focus navigation on the grid.
+ */
 export const GRID_CELL_NAVIGATION_KEYDOWN = 'cellNavigationKeyDown';
+
 export const GRID_CELL_FOCUS = 'cellCellFocus';
 export const GRID_CELL_DRAG_START = 'cellDragStart';
 export const GRID_CELL_DRAG_ENTER = 'cellDragEnter';

to get the description in TypeScript, and then #1213 for rendering the information on the documentation. This also a bit related to #1383.

Motivation 🔦

Remove confusion, and avoid the need to search deep in the codebase. #1515 (comment)