Test downstream projects

Add messaging docs for optional features and kernel subshells #1252

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Run time

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Job	Run time
jupyter_kernel_test	1m 57s
jupyter_server	3m 44s
nbclient	1m 52s
ipykernel	44s
papermill	1m 56s
nbconvert	2m 19s
qtconsole	1m 37s
downstreams_check	7s
	14m 16s