Merge and revamp SQL tutorials

[p7nov]

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.

[andreyaksenov]

It also might be useful to make examples more human-readable. For example, replace generic table2/t6/whatever names and corresponding values ([0, '!!!', "\0", null, 0, '!!!']) with smth more understandable (examples here look much clearer https://mariadb.com/kb/en/foreign-keys/).

UPD: Added several runnable samples with human-readable data, for example:

• https://github.com/tarantool/doc/blob/latest/doc/code_snippets/test/sql/primary_key_table_constraint_test.lua
• https://github.com/tarantool/doc/blob/latest/doc/code_snippets/test/sql/foreign_key_table_constraint_test.lua
• ... and other in the test/sql folder.