🎨 Use `nipype.sphinxext.plot_workflow` instead of `exec` to include workflow diagrams in docstrings

[shnizzedy]

Related problem

Not really a problem, but something that could be improved.

Currently we run exec:: directives in docstrings to generate workflow graphs for Nipype workflows. This works fine but displays a blank line of output.

Nipype has a Sphinx extension to build the graphs with less code in the docstrings and without the blank line of output displaying.

Proposed feature

Include 'nipype.sphinxext.plot_workflow' in https://github.com/FCP-INDI/fcp-indi.github.com/blob/565c9e02850f09a57fb2971fa4ed365819657d32/docs/_sources/conf.py#L118-L128

Replace exec:: workflow directives in docstrings with pithier workflow:: directives.