Added CostFn and Plot to the documentation

[gjhuizing]

Refers to issue #265

[codecov-commenter]

Codecov Report

Merging #288 (fcb3b80) into main (de2c1d7) will decrease coverage by 1.27%.
The diff coverage is n/a.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #288      +/-   ##
==========================================
- Coverage   87.19%   85.93%   -1.27%     
==========================================
  Files          50       52       +2     
  Lines        5334     5587     +253     
  Branches      806      852      +46     
==========================================
+ Hits         4651     4801     +150     
- Misses        563      663     +100     
- Partials      120      123       +3     

Impacted Files	Coverage Δ
src/ott/problems/linear/potentials.py	63.81% <0.00%> (-26.39%)	⬇️
src/ott/solvers/nn/neuraldual.py	55.96% <0.00%> (-14.41%)	⬇️
src/ott/types.py	70.00% <0.00%> (ø)
src/ott/utils.py	86.20% <0.00%> (ø)
src/ott/math/utils.py	84.09% <0.00%> (ø)
src/ott/geometry/graph.py	31.87% <0.00%> (ø)
src/ott/geometry/segment.py	100.00% <0.00%> (ø)
src/ott/solvers/nn/layers.py	88.46% <0.00%> (ø)
src/ott/math/decomposition.py	43.20% <0.00%> (ø)
src/ott/solvers/linear/sinkhorn.py	95.19% <0.00%> (ø)
... and 15 more