Research: SQL documentation for picodata.io

Sbroad SQL documentation is not perfect (no documentation in the world is) and needs improvements. We have two referencing examples of the SQL database tutorials: Tarantool and PostgreSQL. Both documentations have a common structure (and we should implement something similar).

• preface (tnt, pg) - what this product is about, where to report about problems and get help.
• getting started (tnt, pg) - how to configure the initial environment to execute SQL statements
• SQL guide - tutorial how to write SQL queries for beginners (tnt, pg) and professionals (tnt, pg)
• internals (pg) - how it works under hood
• reference (tnt, pg) - a detailed explanation of each SQL statement with the usage examples and limitations

At the moment the only articles in our documentation that I like are:

1. internals of the query execution - good enough
2. getting started - though I think it should contain screenshots for dummies.

All other articles should be redesigned from the scratch (btw, I believe that interactive railroad diagrams should be implemented as a part of reference chapter). The goal of the current research is to approve the structure of the documentation (@kostja) that can be decomposed into multiple issues on the board.