Add FAQ & Troubleshooting section to Destination Actions docs

[bobbyatsegment]

Proposed changes

These docs do not currently have a FAQ & Troubleshooting section - added this to cover two scenarios that customers have written in about.

1. Validation error when using the Event Tester - a validation error with the description "You may not have any subscriptions that match this event" will surface when sending an event with an Actions destination's event tester that does not match the trigger of any configured and enabled mappings:
   

Added a bullet that covers this error along with options for resolving it (creating a new mapping to handle the event being tested or using a test event that matches an existing mapping)

2. Multiple mappings triggered by the same event - this has been asked by several customers, and couldn't find another location for this information, so created a bullet for this as well.

For example, in my own workspace's GA4 destination instance I have two custom event mappings, one generates an event called "event_one", and the other generates an event called "event_two". Both are configured to trigger on an event called "Segment Test Event Name":

By using the destination's event tester, I can see that multiple requests are made to Google, and can cycle through them by clicking the arrows at the top by the "Event Lifecycle" heading:

First request sent for custom event event_one:

Second request sent for custom event event_two:

Merge timing

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