Doc: Add citations/year figure

[dweindl]

Add citations/year figure. The figure is created automatically during the sphinx-build.

👀 https://amici--3148.org.readthedocs.build/en/3148/references.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.70%. Comparing base (de6d34f) to head (c5556d8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3148      +/-   ##
==========================================
- Coverage   77.36%   76.70%   -0.66%     
==========================================
  Files         317      317              
  Lines       20628    20628              
  Branches     1483     1483              
==========================================
- Hits        15958    15822     -136     
- Misses       4662     4798     +136     
  Partials        8        8              

Flag	Coverage Δ
cpp	70.42% <ø> (-0.02%)	⬇️
cpp_python	37.14% <ø> (ø)
petab	47.16% <ø> (ø)
python	68.89% <ø> (-0.01%)	⬇️
sbmlsuite-jax	?

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see 7 files with indirect coverage changes

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