docs: add BayesianKernelDensityClassifier example to mixture methods guide (#290)

[C3L0]

Description

• Added an example code for BayesianKernelDensityClassifier in docs/_scripts/mixture-methods.py.
• Generated and saved bayes-kde-classifier.png in docs/_static/mixture-methods/.
• Updated docs/user-guide/mixture-methods.md with explanatory text, a code block import, a plot figure, and an API link reference.

Fixes #290

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the style guidelines (ruff)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (also to the readme.md)
• I have added tests that prove my fix is effective or that my feature works
• I have added tests to check whether the new feature adheres to the sklearn convention
• New and existing unit tests pass locally with my changes

[koaning]

Cool, thanks for adding it!