DOC: Sphinx 3 compatibility

[effigies]

Summary

The :workflow: directive uses a deprecated API that was removed in Sphinx 3.

Fixes #3205.

List of changes proposed in this PR (pull-request)

• Refactors the workflow directive.
• Adds https://github.com/marketplace/actions/run-circleci-artifacts-redirector to make reviewing documentation changes more straightforward.

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[effigies]

It looks like we don't use this directive anywhere, so there are no docs to check. Using nipreps/fmriprep#2092 to test.

[codecov]

Codecov Report

Merging #3206 into master will increase coverage by 0.03%.
The diff coverage is 7.50%.

@@            Coverage Diff             @@
##           master    #3206      +/-   ##
==========================================
+ Coverage   64.85%   64.89%   +0.03%     
==========================================
  Files         301      301              
  Lines       39715    39717       +2     
  Branches     5268     5268              
==========================================
+ Hits        25759    25776      +17     
+ Misses      12894    12884      -10     
+ Partials     1062     1057       -5     

Flag	Coverage Δ
#unittests	64.89% <7.50%> (+0.03%)	⬆️

Impacted Files	Coverage Δ
nipype/sphinxext/plot_workflow.py	14.95% <7.50%> (+1.16%)	⬆️
nipype/pipeline/plugins/base.py	59.72% <0.00%> (+1.64%)	⬆️
nipype/pipeline/plugins/legacymultiproc.py	68.81% <0.00%> (+3.46%)	⬆️

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[satra]

i think we were using it in nipype workflows which has moved to niflows.

[effigies]

I don't think we're building the docs there, yet.

Functionality is preserved using the new API with Sphinx 2: https://13143-53175327-gh.circle-artifacts.com//0/tmp/src/fmriprep/docs/_build/html/workflows.html

[effigies]

Also with Sphinx 3: https://13149-53175327-gh.circle-artifacts.com//0/tmp/src/fmriprep/docs/_build/html/workflows.html

[effigies]

I removed this line, and then black had to have this file.

We're now at least exercising the code in doc builds again, even if it won't show up in coverage. I'm going to merge.