Update documentation for chat notifications on deployment events

Update configuration screenshots for Slack and Mattermost
Update list of events for Mattermost

doc/user/project/integrations/mattermost.md

@@ -27,9 +27,11 @@ There, you will see a checkbox with the following events that can be triggered:
 - Confidential issue
 - Merge request
 - Note
+- Confidential note
 - Tag push
 - Pipeline
 - Wiki page
+- Deployment
 
 Below each of these event checkboxes, you have an input field to enter
 which Mattermost channel you want to send that event message. Enter your preferred channel handle (the hash sign `#` is optional).