update doc to include task_mode for DaskVine

[BarrySlyDelgado]

Proposed Changes

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Mention relevant issues and pull requests as needed.

Merge Checklist

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• make test Run local tests prior to pushing.
• make format Format source code to comply with lint policies. Note that some lint errors can only be resolved manually (e.g., Python)
• make lint Run lint on source code prior to pushing.
• Manual Update Update the manual to reflect user-visible changes.
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• Product Labels Select a github label for the product: TaskVine, Makeflow, etc.
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[BarrySlyDelgado]

For #3738. Adds task_mode to DaskVine documentation.