Fix TOC in Theory

[RemDelaporteMathurin]

Proposed changes

This PR fixes the bug in the ToC introduced in #664 by updating the version of sphinx-book-theme and sphinx

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 ✅

Comparison is base (d8a2063) 98.91% compared to head (2ad9b4a) 98.91%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #670   +/-   ##
=======================================
  Coverage   98.91%   98.91%           
=======================================
  Files          57       57           
  Lines        2128     2128           
=======================================
  Hits         2105     2105           
  Misses         23       23           

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[jhdark]

LGTM