Fix API docs

[RemDelaporteMathurin]

Proposed changes

The docs won't compile in RTD because of the src structure.

Types of changes

What types of changes does your code introduce to FESTIM?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.69%. Comparing base (0c78549) to head (e8cafcc).
Report is 3 commits behind head on fenicsx.

Additional details and impacted files

@@           Coverage Diff            @@
##           fenicsx     #905   +/-   ##
========================================
  Coverage    97.69%   97.69%           
========================================
  Files           43       43           
  Lines         2042     2042           
========================================
  Hits          1995     1995           
  Misses          47       47           

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[RemDelaporteMathurin]

The docs now compile correctly: