Update the Transactions section

[rindblack15]

Related dev. issue(s):

Product: Tarantool
Since: 2.10
Root document: https://www.tarantool.io/en/doc/latest/book/box/atomic/
SME: @ Mons

Details

Use the material to restructure and rewrite the transaction section to better suit the user's needs.

Definition of done

• The structure of the transaction section has been changed.
• The information has been updated.

Related with #2812, #2793.

[patiencedaur]

Consider including the content of the first-level "Streams" section in the new "Transactions" section.
The original decision to put streams on the first level was was based on a marketing premise, because streams were a key feature of the 2.10.0.beta2 release. Now the hype is gone, and we can safely put this content among the other transaction-related information where it belongs.