Rework BBv2 documentation

[René Rivera]

There's lots of complaints about the BBv2 documentation Artyom had the good recommendation to structure it as:

1. General Concepts:
   1. Syntax
   2. Rules
   3. etc.

2. Tutorial How to do common stuff:
   1. Build a program
   2. Build a library
   3. Build a test
   4. Configure things conditionally
   5. Create extensions
   6. And so on.

Pass on common features of Autotools, CMake, SCons and make sure users can easily figure out how to do.

3. Full reference documentation:
   1. Properties
   2. Modules
   3. Rules

And so on. This is even more important then tutorial because user and search for specific functions rules if they documented but he can't if there is no docs.

As example I can provide ICU, their tutorials are very poor and basic but their reference docs are really great.