DOC: Tell user about the run time of the `pediatric_airway_atlas` pipeline

The user may be asked by `VPAW` (specifically, the underlying `3D Slicer`) to choose between `Wait` and `Force Quit` during a long `pediatric_airway_atlas` pipeline run and this may be confusing. To reduce the confusion we might choose one or more among

1. Raise an info dialogue when the user hits "run" that says "this may take a while, please let the program do its thing" and have the dialogue have a checkbox to not show it again.
2. Update the web-page documentation to indicate "be aware that this can take a while, just let it do its thing"
3. Dump the console output from `pediatric_airway_atlas` into a `QTextEdit` so that the user sees that something is happening and also sees whatever errors might be generated by the pipeline.

Credit to @ebrahimebrahim for formulating most of the issue text.

The issue identifies the pediatric_airway_atlas pipeline and the VPAW/3D Slicer run flow, but names no files or tests. First decide whether the intended outcome is a run-time dialog, web-page guidance, console output in a QTextEdit, or a combination, then locate the run entry point and relevant documentation. Done means the chosen behavior reduces confusion during long runs.

Written by the indexing model from the issue text.