updated explainer

[snehagarwal1]

Update gamepad raw input event explainer based on prototype implementation
This CL updates the explainer to reflect insights gained from the
prototype implementation:

Clarify event dispatch model:

• Event inherits from Gamepad object rather than the event itself
• Removed Gamepad snapshot attribute
• There is no longer a need for a Gamepad live object therefore Events dispatch on Window object instead of Gamepad (matching implemented IDL)
• Update examples to reflect actual event target.

Align examples with actual IDL:

• Added touchesChanged property in the event.
• Corrected name of the event.