Test documentation code blocks

[RemDelaporteMathurin]

Proposed changes

This is closing #718 by making use of doctest to test code blocks in the documentation. That will make the docs more reliable in the future as changes will be found immediately! Plus it helped fixing existing issues in the documentation.....

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
• [x] Documentation Update (if none of the other choices apply)
• [ ] New tests

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.53%. Comparing base (36da67b) to head (48842dd). Report is 175 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #780 +/- ## ======================================= Coverage 99.53% 99.53% ======================================= Files 59 59 Lines 2577 2577 ======================================= Hits 2565 2565 Misses 12 12 ```

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