take docstrings outside discretiser base class

[solegalli]

No description provided.

[solegalli]

FYI @glevv

Please rebase

[codecov]

Codecov Report

Merging #604 (f8ab8d0) into main (2e3bc87) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #604   +/-   ##
=======================================
  Coverage   97.80%   97.80%           
=======================================
  Files          91       92    +1     
  Lines        3507     3510    +3     
  Branches      684      684           
=======================================
+ Hits         3430     3433    +3     
  Misses         29       29           
  Partials       48       48           

Impacted Files	Coverage Δ
feature_engine/discretisation/base_discretiser.py	90.47% <ø> (-1.84%)	⬇️
feature_engine/_docstrings/fit_attributes.py	100.00% <100.00%> (ø)
...engine/_docstrings/init_parameters/discretisers.py	100.00% <100.00%> (ø)
feature_engine/_docstrings/methods.py	100.00% <100.00%> (ø)
feature_engine/discretisation/arbitrary.py	87.50% <100.00%> (+0.32%)	⬆️
feature_engine/discretisation/equal_frequency.py	100.00% <100.00%> (ø)
feature_engine/discretisation/equal_width.py	100.00% <100.00%> (ø)

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