Use a new configuration approach for bootstrapping a database in SQL tutorials

[xuniq]

Fixes #3672

What has been done:

• Updated Improving MySQL with Tarantool page: https://github.com/tarantool/doc/pull/3891/files#diff-c243191ef89b4e031a1f3631024a625cf0e5fb513791f0ffccde63e351bc33a9L1

• Updated SQL tutorial page: https://github.com/tarantool/doc/pull/3891/files#diff-95e6dc7b9efa335f728ba27ba49e8b0684201521ed5c2de7747d853e72b154e4L2

• Added mysql directory to the code-snippets/snippets/config folder (used in the Improving MySQL with Tarantool tutorial). The directory contains .lua file, config.yaml, and instances.yml.
  https://github.com/tarantool/doc/pull/3891/files#diff-3fd0774b19b03018c1e7f4d5d3899e89811433354b0ab05f00d97b5f8c9a4c60R1

Staging:

• https://docs.d.tarantool.io/en/doc/gh-3672/how-to/sql/sql_tutorial/
• https://docs.d.tarantool.io/en/doc/gh-3672/how-to/sql/improving_mysql/

[Totktonada]

I think that this part should be reviewed with @psergee.

AFAIU, we want to create a system wide instance and use tarantoolctl's layout here. AFAIK, tt supports it. However, it also may provide better ways to install things into a system.

[psergee]

This approach does not seem to work. Yes, tt can work with tarantoolctl layout for single-script instances. But here we have a config. To make tt work with config.y[a]ml and instances.y[a]ml, these files must be placed in a sub-directory of instances.enabled:

• Instead of an example.lua, a user should create /etc/tarantool/instances.available/mysql/init.lua
• Create config.yml and instances.yml in the mysql dir from the step above
• Create a link in instances.enabled to this mysql directory.

[Totktonada]

TBH, I don't like this manual. A lot of 'put files here and there' actions are cumbersome. We should provide deb/rpm packages for such purposes and the commands should be mostly apt-get install <...> plus edit config plus systemctl start <...>.

However, I don't mind a slight update to sync it with recent tarantool versions and the tooling around. If the new steps are working, they're likely OK.

Please, review with tt guys and proceed.