readthedocs: install graphviz

[mr-c]

and other docs cleanups

[codecov]

Codecov Report

Merging #1794 (aff6c1f) into main (126be96) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1794   +/-   ##
=======================================
  Coverage   83.48%   83.49%           
=======================================
  Files          44       44           
  Lines        8097     8099    +2     
  Branches     2217     2217           
=======================================
+ Hits         6760     6762    +2     
  Misses        855      855           
  Partials      482      482           

Impacted Files	Coverage Δ
cwltool/command_line_tool.py	87.21% <ø> (ø)
cwltool/context.py	100.00% <ø> (ø)
cwltool/process.py	92.74% <ø> (ø)
cwltool/singularity.py	70.56% <ø> (ø)
cwltool/argparser.py	95.27% <100.00%> (+0.03%)	⬆️

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