Add code examples in preprocessing and wrappers modules' docstrings

[datacubeR]

Added examples for the following modules (both in the same PR because they are just a few classes).

• Wrappers: SklearnTransformerWrapper
• Preprocessing: MatchCategories and MatchVariables.

[codecov[bot]]

Codecov Report

Merging #643 (0bcdb9c) into main (c2f6152) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 0bcdb9c differs from pull request most recent head cc12391. Consider uploading reports for the commit cc12391 to get more accurate results

@@           Coverage Diff           @@
##             main     #643   +/-   ##
=======================================
  Coverage   97.91%   97.91%           
=======================================
  Files         100      100           
  Lines        3748     3748           
  Branches      726      726           
=======================================
  Hits         3670     3670           
  Misses         29       29           
  Partials       49       49           

Impacted Files	Coverage Δ
feature_engine/preprocessing/match_categories.py	93.02% <ø> (ø)
feature_engine/preprocessing/match_columns.py	97.91% <ø> (ø)
feature_engine/wrappers/wrappers.py	97.46% <ø> (ø)

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