Merge and revamp SQL tutorials

There are two SQL tutorials in Tarantool docs:

• SQL beginner's guide
• SQL tutorial

Issues:

• they are both huge
• they are repeating each other
• there is no clear difference between them from the reader's point of view
• the beginner's guide provides a lot of entry-level SQL content that is not a matter of Tarantool docs (for example, what is a table).

This leads to problems for both the doc team and the readers:

• readers don't know what's the difference and which guide should they choose
• the doc team wastes a lot of effort on supporting these pages when new SQL features come out or the behavior changes
• it's a duplicating content that we (teamD, teamX, translators) have to maintain

Proposed solution: merge the two pages in a single end-to-end tutorial on using SQL in Tarantool, leaving only the relevant content.

Start by comparing the two named pages: SQL beginner's guide and SQL tutorial. Identify repeated entry-level SQL material and Tarantool-specific content, then consolidate the relevant material into one end-to-end SQL-in-Tarantool tutorial. Done means the duplicate pages are merged or redirected and the resulting documentation has a clear scope for readers and maintainers.