Better handle displaying streams and progress bars

[maximlt]

We have some issues when building the site dealing with outputs to stdout/stderr (see screenshots below):

1. streams are displayed on multiple cells
2. too many progress bar outputs displayed

For 1, I just found out about he nb_merge_streams option of MyST-NB that is False by default. We could try to set it to True and see how it goes. See https://myst-nb.readthedocs.io/en/latest/render/format_code_cells.html#group-into-single-streams.

Not sure for 2 but that's definitely been annoying and has forced us to silence progress bars. For example, in #274 I set INTAKE_CACHE_PROGRESS to false to disable the progress bar automatically displayed by intake when it downloads a dataset (only during the notebook evaluation for building the site).

Streams on multiple cells:

Progress bar output:

Note that this worked well on the old examples.pyviz.org, that used the NotebookDirective (implemented pre MyST-NB, based on nbconvert?) and a custom theme: