DOC: Fix non-standard Input/Output docstring section management

[jhlegarreta]

Changes proposed in this pull request

Fix non-standard Input/Output docstring section management: map Inputs and Outputs to params_style and returns_style, respectively, for napoleon custom sections (instead of mapping them to Parameters).

Documentation:
https://www.sphinx-doc.org/en/master/usage/extensions/napoleon.html#confval-napoleon_custom_sections

Documentation that should be reviewed

Ideally, build locally to check it this makes the trick and check e.g. the confound computation documentation https://petprep.readthedocs.io/en/latest/api.html#calculate-pet-confounds.

[jhlegarreta]

Have not checked that this works.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.73%. Comparing base (4311dcd) to head (dd7b67d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #105   +/-   ##
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  Coverage   78.73%   78.73%           
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  Files          80       80           
  Lines        5870     5870           
  Branches      591      591           
=======================================
  Hits         4622     4622           
  Misses       1094     1094           
  Partials      154      154           

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