move selection docstrings to docstrings module

[solegalli]

closes #649

[codecov]

Codecov Report

Merging #678 (b65dddd) into main (e73772d) will increase coverage by 0.06%.
The diff coverage is 99.24%.

@@            Coverage Diff             @@
##             main     #678      +/-   ##
==========================================
+ Coverage   97.91%   97.98%   +0.06%     
==========================================
  Files         100      100              
  Lines        3748     3827      +79     
  Branches      726      751      +25     
==========================================
+ Hits         3670     3750      +80     
+ Misses         29       28       -1     
  Partials       49       49              

Impacted Files	Coverage Δ
feature_engine/_docstrings/selection/_docstring.py	100.00% <ø> (ø)
feature_engine/datetime/datetime_subtraction.py	94.66% <ø> (ø)
feature_engine/tags.py	100.00% <ø> (ø)
feature_engine/selection/drop_psi_features.py	99.40% <98.57%> (+1.08%)	⬆️
feature_engine/creation/relative_features.py	100.00% <100.00%> (ø)
feature_engine/encoding/rare_label.py	100.00% <100.00%> (ø)
feature_engine/imputation/drop_missing_data.py	100.00% <100.00%> (ø)
feature_engine/selection/drop_constant_features.py	100.00% <100.00%> (ø)
...ature_engine/selection/drop_correlated_features.py	90.00% <100.00%> (ø)
...eature_engine/selection/drop_duplicate_features.py	95.45% <100.00%> (ø)
... and 10 more

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