Skip task in example_dynamodb_to_s3.py

[ferruzzi]

backup_db_to_point_in_time_incremental_export requires at least a 15-minute window of available data, causing the system tests to fail. Moving it into a branching task group will let us keep the code snippet in the docs without actually running the task in the test.

Workaround for the issue presented in #41304. If someone wants to take on an actual fix, tag me and I'll review it, but for now I want to get the test running again. Branching operators are a bit confusing, see how we did it in the Bedrock system test (here) if you want to see some existing working examples.

TLDR of the issue:

error: "from" time can't be equal to "to" time
tried: so I set "from" to export_time - 1_minute
error: "from" can't be less than table creation time
tried: revert the above and set "to" to be latest_export_time + 1_minute
error: Difference between "from" time and "to" is less than 15 minutes
tried: revert the above and set "to" to be latest_export_time + 16_minutes
error: "to" time cannot be greater than the current time
tried: revert the above and set "to" time to utcnow()
error: this gets set at parse time and "to" time is now in the past

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