docs: bump version, prepare release.

[janfb]

• condense and refine change log
• change names to GH handles
• stick with version 0.23?
• do we need a short tutorial for the new density estimator API and the new training interface?
• update all license comments to Apache 2.0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.16%. Comparing base (3b9be6a) to head (fbd366b).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1170      +/-   ##
==========================================
- Coverage   86.08%   78.16%   -7.93%     
==========================================
  Files         114      117       +3     
  Lines        8662     8679      +17     
==========================================
- Hits         7457     6784     -673     
- Misses       1205     1895     +690     

Flag	Coverage Δ
unittests	78.16% <100.00%> (-7.93%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
sbi/__version__.py	100.00% <100.00%> (ø)
sbi/diagnostics/lc2st.py	95.97% <ø> (+0.02%)	⬆️
sbi/inference/posteriors/ensemble_posterior.py	50.00% <ø> (-37.97%)	⬇️
.../neural_nets/estimators/mixed_density_estimator.py	98.11% <ø> (ø)
sbi/neural_nets/factory.py	90.90% <ø> (ø)
sbi/neural_nets/net_builders/categorial.py	94.73% <ø> (ø)
sbi/sbi_types.py	100.00% <ø> (ø)

... and 63 files with indirect coverage changes

[michaeldeistler]

Thanks a ton! I am wondering if we should split Major Changes and New Features into two subcategories (Major Changes, and New Features)?

[janfb]

Looks good!