Add note about sending Track events with a custom Group Key to Mixpanel

[joeynmq]

Proposed changes

Currently, our Mixpanel (Actions) destination uses $group_id as the standard Group Key for Track events. Integration code here:

• https://github.com/segmentio/action-destinations/blob/main/packages/destination-actions/src/destinations/mixpanel/trackEvent/functions.ts#L40

 
However, a customer asked about how can they send a custom Group Key in this case. And I discovered a workaround for this - all they need to do is to include their custom Group Key as a property of Track events.

For example (I'm using company_id as the custom Group Key):

analytics.track('Example Event', { company_id: 'group1' });

Merge timing

ASAP once approved

[github-actions]

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