update existing documentation with high level architecture diagram for reactor-netty.

[gurudatta11]

Currently I don't see any good documentation regarding reactor-netty architecture for beginners or whoever already using reactor-netty.
I see documentation on how to consume it (which is helpful) for developers.
https://projectreactor.io/docs/netty/release/reference/index.html

Motivation

Desired solution

Please provide high level architecture documentation of reactor-netty (including connection pool as well).
With diagrams is helpful

Considered alternatives

Additional context

[ctlove0523]

@violetagg
it would be appreciate it there is a description of reactor netty' thread model