Add A/B Testing page

[kshkalenkova]

I would like to create a new page for A/B Testing under the Machine Learning section of LOST.

The page will include:

• Introduction to A/B testing
• Assumptions and considerations
• Python implementation example
• Interpretation of results
• Common pitfalls and limitations

I plan to follow the existing LOST page structure and contribution guidelines.

[NickCH-K]

Sounds good, go for it! Let us know when you have it ready.