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

[Leengit]

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.