Add flake8-docstrings to CI

[mauicv]

What is this?

See this note!

This PR adds flake-docstrings to lint docstrings. Originally I was setting the convention option to NumPy however flake-docstrings excludes D417-Missing argument descriptions in the docstring for NumPy. Instead, we disable the codes manually.

I've also fixed some faulty docstring formatting, specifically in the return statements for the detectors. Fixes #759

Notes

1. Flake disables certain checks by default. Specifically: E121, E123, E126, E226, E24, E704, W503, W504 I've kept these defaults with the exception of E226 missing whitespace around arithmetic operator.
2. Flake8-docstrings consider the following list of rules exceptions to the NumPy convection. D203, D212, D213, D402, D413, D415, D416. I've manually disabled these in order to enable D107 and D417. D107 enforces __init__'s to have docstrings and D417 enforces all parameters to be listed in docstring.
3. The following are temporarily disabled. Will be re-enabled in a future PR: D1, D401, D205, D400. These have been added to ignores because implementing them here would be a very big job. I suggest we find some way of doing this iteratively!
4. Ignore D403 due to this issue
5. Ignore D208 due to conflicts with returns statement formatting.

[codecov-commenter]

Codecov Report

Merging #748 (20cf72f) into master (5a38fc2) will decrease coverage by 0.57%.
The diff coverage is 90.81%.

❗ Current head 20cf72f differs from pull request most recent head 149f08b. Consider uploading reports for the commit 149f08b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #748      +/-   ##
==========================================
- Coverage   81.38%   80.81%   -0.57%     
==========================================
  Files         154      144      -10     
  Lines       10007     9619     -388     
==========================================
- Hits         8144     7774     -370     
+ Misses       1863     1845      -18     

Flag	Coverage Δ
macos-latest-3.9	77.42% <88.77%> (?)
ubuntu-latest-3.10	80.72% <90.81%> (?)
ubuntu-latest-3.7	80.62% <90.81%> (?)
ubuntu-latest-3.9	80.67% <90.81%> (?)
windows-latest-3.9	77.42% <88.77%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
alibi_detect/ad/adversarialae.py	62.72% <ø> (-0.67%)	⬇️
alibi_detect/ad/model_distillation.py	91.52% <ø> (-0.28%)	⬇️
alibi_detect/base.py	85.45% <ø> (ø)
alibi_detect/cd/_domain_clf.py	88.23% <ø> (ø)
alibi_detect/cd/base.py	90.97% <ø> (-0.22%)	⬇️
alibi_detect/cd/base_online.py	88.23% <ø> (ø)
alibi_detect/cd/chisquare.py	97.14% <ø> (ø)
alibi_detect/cd/classifier.py	100.00% <ø> (ø)
alibi_detect/cd/context_aware.py	97.43% <ø> (ø)
alibi_detect/cd/learned_kernel.py	100.00% <ø> (ø)
... and 76 more

... and 16 files with indirect coverage changes