docs: ZModel - Under the hood.

[zmzlois]

As discussed with @ymc9, here are some thoughts on the ZenStack explaination article and the goal is to edit it as easily understood as possible without confusion.

How Does ZenStack Work Under the Hood?

1. Should this article act as the most comprehensive explaination of how ZenStack works?
2. Specify ZenStack created ZModel so people don't need to Google it like "If you want to understand ZModel, the DSL we created for ZenStack, you can find the most comprehensive explanation here, we will add more as questions come in on the way." (I personally did try to google ZModel and tried to find out what it is)
3. Also added some extra explanation for ease of reading to make sure someone has only touched frontend would understand things easily.

I have left my comments and edit suggestions in this commit, 2fcc469 , WIP