[3pt] Intro and explanation of Streams feature

# Feature name

Fill in whatever you know at the moment.

**Product:** Tarantool
**Since:** 2.10.0-beta1
**Audience/target:** connector devs, external devs
**Root document:** tbd
**SME:** @EvgenyMekhanik 
**Peer reviewer:** @ tbd

# Details

Introductory document :

* What's in the new feature
* What issues it helps resolve
* Who should use it

# Definition of done