[GSK-1274] Refactoring of demo

[rabah-khalek]

fixes: https://linear.app/giskard/issue/GSK-1274/giskarddemotitanic-is-broken

I refactored titanic classification and linear regression demos, keeping in __init__.py only the factory methods (without breaking our docs).

[linear]

GSK-1274 giskard.demo.titanic() is broken

• it’s because titanic() is in demo.__init__.py- which is not a very good practice, init code should be reserved for import, not actual business logic
• And demo isn’t part of _all_ in  giskard.__init__.py

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
4 Code Smells

68.3% Coverage
0.0% Duplication