Add dask as dependency

Realized that it's not listed as a dependency even though it was being used in some of the code. This is needed when creating the environments for each proposal.

(On that note, one can now use scs-create-toolbox-env --proposal <PROPOSAL> to generate the environment.)

@lleguy @mercadil

doc/index.rst

@@ -108,6 +108,7 @@ Documentation contents
    :maxdepth: 2
 
    howtos.rst
+   maintainers.rst
 
 
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