xgboost-based app

[Abdullahi-Ahmed]

Closes #15

Changes proposed (Describe Changes)

An XGBoost-based app is added to the existing UnionML documentation, including an example of how to use UnionML to train and deploy an XGBoost model for classification tasks.

Check List (Check all the applicable boxes)

• My code follows the code style of this project.
• My change requires changes to the documentation.
• I have updated the documentation accordingly.
• All new and existing tests passed.
• This PR does not contain plagiarized content.
• The title of my pull request is a short description of the requested changes.

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