[DOC] Create an FAQ for Merlin Models

[EvenOldridge]

Let's create an FAQ highlighting:

• What does Merlin models do?
• Why should I use Merlin models?
• What are blocks?
• What is a block context?

etc

@karlhigley and @jperez999 have a starting point of some questions.

[rnyak]

I think we can also add some use-case dependent questions and their answers (linking to the docs or available example nbs) coming from users such as:

• how to do item-item recommendations
• how to generate a schema file
• how to save and load back a model, and do local prediction etc etc

[bschifferer]

@karlhigley @jperez999 have you started some example questions?