Feature/new docstyle

[sblauth]

This PR switches cashocs docstyle to the pydata sphinx theme.

Closes #98

[codecov-commenter]

Codecov Report

Merging #122 (b76d60f) into main (880b227) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
- Coverage   93.51%   93.50%   -0.00%     
==========================================
  Files          66       65       -1     
  Lines        5651     5649       -2     
==========================================
- Hits         5284     5282       -2     
  Misses        367      367              

Impacted Files	Coverage Δ
cashocs/_optimization/cost_functional.py	99.24% <ø> (ø)
...cs/_optimization/line_search/armijo_line_search.py	98.25% <ø> (ø)
...optimization/line_search/polynomial_line_search.py	81.32% <ø> (ø)
...ization/optimal_control/optimal_control_problem.py	100.00% <ø> (ø)
...zation/optimization_algorithms/gradient_descent.py	100.00% <ø> (ø)
...cs/_optimization/optimization_algorithms/l_bfgs.py	99.29% <ø> (ø)
...shocs/_optimization/optimization_algorithms/ncg.py	100.00% <ø> (ø)
...cs/_optimization/optimization_algorithms/newton.py	80.56% <ø> (ø)
...n/shape_optimization/shape_optimization_problem.py	96.95% <ø> (ø)
cashocs/_pde_problems/adjoint_problem.py	100.00% <ø> (ø)
... and 23 more

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