chore(rtd): Build API docs

[effigies]

Missed in previous PR.

[effigies]

That's better: https://nibabel-draft--1399.org.readthedocs.build/en/1399/api.html#api

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.37%. Comparing base (2a64669) to head (e61cb54).
Report is 67 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1399   +/-   ##
=======================================
  Coverage   95.37%   95.37%           
=======================================
  Files         207      207           
  Lines       29672    29672           
  Branches     3018     3018           
=======================================
  Hits        28300    28300           
  Misses        934      934           
  Partials      438      438           

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