add transformer that combines features with decision trees

[solegalli]

closes #107 superseeds #454

• [x] add example to docstring
• [x] add user guide
• [x] add transformer to readme and index

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.31544% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 98.22%. Comparing base (389b515) to head (ca65970).

:exclamation: Current head ca65970 differs from pull request most recent head b028369

Please upload reports for the commit b028369 to get more accurate results.

Files	Patch %	Lines
feature_engine/creation/decision_tree_features.py	97.16%	2 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #766 +/- ## ========================================== - Coverage 98.25% 98.22% -0.04% ========================================== Files 105 107 +2 Lines 4137 4283 +146 Branches 817 854 +37 ========================================== + Hits 4065 4207 +142 - Misses 28 30 +2 - Partials 44 46 +2 ```

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