Improve docs organization

[nicolashuber]

I suggest organizing the documentation in the following topics:

• Getting started
  • Why Aztlan?: quickly describe the features
  • Installation
  • Directory structure
• Configuration
  • Environment variables
• Theme development
  • Templates: describe where to keep the theme templates
  • Frontend: describe assets directory and basic commands
  • Backend: describe how to create classes to manipulate WordPress
• Commands: list all Azltan commands

The idea is create a more friendly documentation. For new users understand the purpose of each directory. I already created the branch issue_1 with my initial ideia.