docs: add page on differentiability

[gdalle]

Fixes #59

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.16%. Comparing base (0f3d2c1) to head (1a0c58d).

❗ There is a different number of reports uploaded between BASE (0f3d2c1) and HEAD (1a0c58d). Click for more details.

HEAD has 74 uploads less than BASE

Flag	BASE (0f3d2c1)	HEAD (1a0c58d)
DI	70	12
DIT	16	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #665       +/-   ##
===========================================
- Coverage   97.60%   71.16%   -26.44%     
===========================================
  Files         112       91       -21     
  Lines        5592     3659     -1933     
===========================================
- Hits         5458     2604     -2854     
- Misses        134     1055      +921     

Flag	Coverage Δ
DI	71.16% <ø> (-27.59%)	⬇️
DIT	?

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