Reduce docs compilation warnings

[MatteoRobbiati]

In this PR I aim to reduce the warnings got compiling the documentation.
Not exactly addressing #1228 but related.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.99%. Comparing base (c598c67) to head (6bcc771).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1402   +/-   ##
=======================================
  Coverage   99.99%   99.99%           
=======================================
  Files          78       78           
  Lines       11191    11191           
=======================================
  Hits        11190    11190           
  Misses          1        1           

Flag	Coverage Δ
unittests	99.99% <ø> (ø)

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