add method to transfrom x and y to classes that drop rows from X, add pipeline to handle transform_x_y

[solegalli]

• [x] add test for transform x y mixin
• [x] fix test window features
• [x] update expanding windows transformer
• [x] add test for expanding windows
• [x] create pipeline
• [x] test pipeline
• [x] expand doc dropna
• [x] expand doc outlier trimmer
• [x] expand doc lag features
• [x] expand doc window features
• [x] expand doc expanding window features
• [x] create doc pipeline

[solegalli]

Tests for new pipeline functionality are done.

I need to add remaining tests to corroborate that the original functionality of the pipeline (inherited from sklearn) also continues to work.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.75000% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 98.18%. Comparing base (1a090ba) to head (946dc5e).

Files	Patch %	Lines
feature_engine/pipeline/pipeline.py	90.82%	7 Missing and 3 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #724 +/- ## ========================================== - Coverage 98.31% 98.18% -0.14% ========================================== Files 103 105 +2 Lines 3928 4072 +144 Branches 764 795 +31 ========================================== + Hits 3862 3998 +136 - Misses 23 29 +6 - Partials 43 45 +2 ```

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