[Task] User Guide for our Examples

Description

We have examples across many different libraries and expecting different level of understanding. As a user, I want to have an inventory / user guide of our examples similar to https://scikit-learn.org/stable/user_guide.html and https://www.tensorflow.org/tutorials or https://www.tensorflow.org/guide

The goal is, if a user asks "how to start with Merlin examples" - it should guide a user through our examples. Which examples would you recommend first and then depending per topic.

Some Sections could be:

• Basics with Merlin (Merlin Models Getting Started + Schema File)
• Training
  -- Ranking Models
  -- Retrieval Models
  -- Session-Based Models
  -- Traditional Models
  -- Multi-GPU Training
• Serving with Merlin
• Feature Engineering with Merlin
• End-To-End Examples
• TensorFlow
• PyTorch
• etc.