add details about zuko density estimators

[janfb]

We did not have any details on how to use zuko density estimators.

This adds details about zuko to the tutorial on density estimators (as requested in the JOSS review).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.48%. Comparing base (6d527f7) to head (3c5ad0e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1387       +/-   ##
===========================================
- Coverage   89.31%   78.48%   -10.84%     
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  Files         119      119               
  Lines        8779     8779               
===========================================
- Hits         7841     6890      -951     
- Misses        938     1889      +951     

Flag	Coverage Δ
unittests	78.48% <ø> (-10.84%)	⬇️

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see 33 files with indirect coverage changes

[michaeldeistler]

Thanks!