Improve the Transactions section

[xuniq]

Product: Tarantool
Root document: https://www.tarantool.io/en/doc/latest/book/box/atomic/
SME: @ Mons

Definition of done

• Split the Transactions page into separate sections.
• Add a separate page with the table of contents in the Transactions section.

Example:
https://www.tarantool.io/en/doc/latest/reference/

[patiencedaur]

Related issue:
https://github.com/tarantool/tarantool.io/issues/1275