Need to update distributed SQL page

Main goal - less text, better structure.

Let's add the following sections:

1. Distributed SQL Indexes
2. Distributed SQL JOINs
3. Memory-only
4. Native Persistence
5. Memory Management - here we should mention that due to Ignite memory management policies, the most used indexes will always end up being cached in memory, even when the persistence is enabled.
6. 3rd Party Databases - here we should mention that Ignite SQL does write-through to 3rd party databases, if needed.