[3pt] Document the new pub/sub system and its events

[patiencedaur]

Epic: Document master discovery
Related dev issues:
tarantool/tarantool#6257 (system)
tarantool/tarantool#6260 (events)

Product: Tarantool
Since: 2.10
Audience/target: dev
Root document: TBD
SME: @ yanshtunder , @ Gerold103

Details

Pub/sub is a notification system for Tarantool events. It is related to one-time subscriptions.

Events that the system will process:

• box.status
• box.election
• box.schema
• (are there more in this quarter?)
  As a reaction to each event, the server sends back specific iproto fields.

Related to #2407 #2409 #2408