Additional buttons in header

[RemDelaporteMathurin]

Proposed changes

This PR fixes #760

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 99.51%. Comparing base (fdfcb17) to head (51fea45).
Report is 284 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #762      +/-   ##
==========================================
+ Coverage   99.48%   99.51%   +0.02%     
==========================================
  Files          58       58              
  Lines        2350     2478     +128     
==========================================
+ Hits         2338     2466     +128     
  Misses         12       12              

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[RemDelaporteMathurin]

The result isn't ideal as it adds the icon button in the sidebar and not the header.

I opened an issue to see if it is possible with the sphinx-book-theme