documentation suggestions

[Dmitriy Setrakyan]

I would like to propose some changes to the Ignite documentation:

   - Under Basic Concepts add the following pages:
      - "How to load data"
      - "SQL vs Scan"
      - "SQL vs Key-Value"
      - more?
   - For persistence section, add the following pages:
      - Write-Ahead-Log (WAL)
         - BTW, I could not find WAL modes anywhere in documentation
      - Main Store (or Checkpointing)
      - Configuration

Thoughts?

D.