Update the list of all webhook events

[Agathe-Brusset]

Please update the list of all webhook events in the documentation or any place where they are mentioned:
SOFT_BOUNCED = "activity.soft_bounced",
HARD_BOUNCED = "activity.hard_bounced",
OPENED = "activity.opened",
OPENED_UNIQUE = "activity.opened_unique",
CLICKED = "activity.clicked",
CLICKED_UNIQUE = "activity.clicked_unique",
UNSUBSCRIBED = "activity.unsubscribed",
SPAM_COMPLIANT = "activity.spam_complaint",
SURVEY_OPENED = "activity.survey_opened",
SURVEY_SUBMITTED = "activity.survey_submitted",
IDENTITY_VERIFIED = "sender_identity.verified",
MAINTENANCE_START = "maintenance.start",
MAINTENANCE_END = "maintenance.end",