Adding trigger event type

[ianabels]

Documenting that a Subscription Group update can also be used to trigger a Campaign. I've only added Subscription Group Update due to SMS team adding functionality to it in SMS-900 but in comparing this docs page to the dashboard, it doesn't appear that this list is comprehensive.

@KellieHawks @lydia-xie (adding Lydia as I believe this is technically Core Messaging even though SMS made the change?)

Pull Request/Issue Resolution

Description of Change:

Came across this when looking to see where the docs needed updating for https://jira.braze.com/browse/SMS-900

Closes #ISSUE_NUMBER_HERE

Is this change associated with a Braze feature/product release?

• Yes (Insert Feature Release Date Here)
• No

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Name	Status	Preview	Comments	Updated
braze-docs-fr	⬜️ Ignored (Inspect)			Jan 18, 2023 at 9:09PM (UTC)