Added hierarchy to user and developer api

[LeonStadelmann]

Addressing #773
Added further collapsable section for user and developer api, by adding the following hierarchy:

User API
└── Biological Problems/
     ├── ...
└── Generic Problems/
     ├── ...
└── Plotting/
     ├── ...
└── Datasets/
     ├── ...
Developer API
└── Backends/
     ├── ...
└── Costs/
     ├── ...
└── Base/
     ├── ...
└── Utils/
     ├── ...

[MUCDK]

great, looks good!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.53%. Comparing base (b55a0e4) to head (3d47a12).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #779      +/-   ##
==========================================
+ Coverage   77.46%   77.53%   +0.06%     
==========================================
  Files          37       38       +1     
  Lines        4363     4376      +13     
  Branches      677      677              
==========================================
+ Hits         3380     3393      +13     
  Misses        657      657              
  Partials      326      326              

Files with missing lines	Coverage Δ
src/moscot/base/problems/birth_death.py	89.62% <ø> (ø)
src/moscot/base/problems/compound_problem.py	75.67% <ø> (ø)
src/moscot/costs/_costs.py	97.14% <ø> (ø)
src/moscot/plotting/_plotting.py	73.80% <ø> (ø)
src/moscot/problems/cross_modality/_mixins.py	76.92% <ø> (ø)
src/moscot/problems/cross_modality/_translation.py	96.07% <ø> (ø)
src/moscot/problems/generic/_generic.py	94.02% <ø> (ø)
src/moscot/problems/generic/_mixins.py	56.52% <ø> (ø)
src/moscot/problems/space/_alignment.py	81.39% <ø> (ø)
src/moscot/problems/space/_mapping.py	96.05% <ø> (ø)
... and 5 more