Updated quickstart guide documentation

[Leo-GG]

Description

Updated quickstart guide to update the documentation of the MAPIE regressor output.

Fixes #494

Type of change

• Documentation

Checklist

• [X] I have read the contributing guidelines
• [] I have updated the HISTORY.rst and AUTHORS.rst files
• [X] Linting passes successfully : make lint
• [X] Typing passes successfully : make type-check
• [X] Unit tests pass successfully : make tests
• [X] Coverage is 100% : make coverage
• [ ] Documentation builds successfully : make doc