Add documentation

[georgeyiasemis]

• Fixes some numpydocs issues
• Adds documentation on how to create your own model

[codecov]

Codecov Report

Patch coverage: 57.14% and project coverage change: -0.02 ⚠️

Comparison is base (3a5e2aa) 83.87% compared to head (e637993) 83.86%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #242      +/-   ##
==========================================
- Coverage   83.87%   83.86%   -0.02%     
==========================================
  Files          91       91              
  Lines        6276     6278       +2     
==========================================
+ Hits         5264     5265       +1     
- Misses       1012     1013       +1     

Impacted Files	Coverage Δ
direct/algorithms/mri_algorithms.py	100.00% <ø> (ø)
direct/data/transforms.py	96.47% <ø> (ø)
direct/nn/jointicnet/jointicnet_engine.py	100.00% <ø> (ø)
direct/utils/__init__.py	77.63% <0.00%> (ø)
direct/data/mri_transforms.py	92.29% <80.00%> (-0.19%)	⬇️

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