Docs fixes

[RemDelaporteMathurin]

Proposed changes

We noticed some bugs in the API section of the docs in #791

Also take the opportunity to remove the numbering from the user guide and other docs improvements.

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 99.55%. Comparing base (dd40851) to head (eef4c44).
Report is 135 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #797   +/-   ##
=======================================
  Coverage   99.55%   99.55%           
=======================================
  Files          61       61           
  Lines        2705     2705           
=======================================
  Hits         2693     2693           
  Misses         12       12           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[KulaginVladimir]

Maybe, you could also add indents to Note (I suppose, should be Notes) here and here?

[RemDelaporteMathurin]

@KulaginVladimir anything else to add? or are we good to merge this?