249 clarification of the symmetry argument in cqr and more general documentation about cqr

[LacombeLouis]

Description

This change adds an example to the documentation that demonstrates the impact of the symmetry parameter in the MapieQuantileRegressor class. The example generates plots for prediction intervals with both symmetry=True and symmetry=False, allowing users to visualize and understand the differences.

Fixes #249.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• [x] Ran the script and visually inspected the generated plots to ensure they display correctly.
• [x] Verified that the explanation accurately reflects the results shown in the plots.

Checklist

• [x] I have read the contributing guidelines
• [x] 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
• [x] Documentation builds successfully : make doc