Add deprecation policy

[dweindl]

As suggested in #2458.

Other opinions?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.63%. Comparing base (043d13a) to head (f261219).
Report is 24 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #2465       +/-   ##
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+ Coverage    36.66%   77.63%   +40.97%     
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  Files          103      324      +221     
  Lines        14881    20927     +6046     
  Branches      1388     1464       +76     
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+ Hits          5456    16247    +10791     
+ Misses        9116     4677     -4439     
+ Partials       309        3      -306     

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cpp	73.39% <ø> (?)
cpp_python	34.09% <ø> (?)
petab	36.66% <ø> (ø)
python	72.18% <ø> (?)

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[dilpath]

Looks good, could also add this outside of the changelog, e.g. https://amici.readthedocs.io/en/latest/development.html

[dweindl]

could also add this outside of the changelog, e.g. https://amici.readthedocs.io/en/latest/development.html

Yes, I wasn't sure where to put it. That document says This document contains information for AMICI developers, not too relevant to regular users., so I don't think it's the right place. In the changelog it's the most visible, but I can also add a separate document on versioning, deprecation, ... policy.