docs: add docstrings in the backends.numpy module

[Saransh-cpp]

I have tried to add docstrings for both developers and users. #194 blocked the addition of examples to some properties, but I will definitely add them once the issue is closed. I have skipped some functions and classes as I couldn't properly understand what they were doing. These are -

• def _getitem
• def _setitem
• class GetItem

Please let me know if I should add some more context in the docstrings or if any changes are required!

Note: The class variables are rendered in the documentation with their values automatically. Hence, I decided not to add docstrings for them, but I would be more than happy to override the automatically rendered stuff!

[codecov-commenter]

Codecov Report

Merging #195 (4c8f63b) into main (5387e5e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #195   +/-   ##
=======================================
  Coverage   82.44%   82.44%           
=======================================
  Files          96       96           
  Lines       10386    10387    +1     
=======================================
+ Hits         8563     8564    +1     
  Misses       1823     1823           

Impacted Files	Coverage Δ
src/vector/backends/numpy.py	77.50% <100.00%> (+0.02%)	⬆️

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[henryiii]

GetItem is a mixin that provides the infrastructure for [] access. These are implementation details so don't think it's critical to have good docs for them. The rest is an improvement!