Add Sphinx docs requirements

[corentincarton]

Summary

• add docs/requirements.txt to list Sphinx dependencies

Testing

• make -C docs html (fails: sphinx-build: command not found)
• pytest -q

[FussyDuck]

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.45%. Comparing base (c467a7a) to head (b59fd02).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #105   +/-   ##
========================================
  Coverage    96.45%   96.45%           
========================================
  Files           14       14           
  Lines          395      395           
========================================
  Hits           381      381           
  Misses          14       14           

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