[Documentation] Add documentation regarding message broker tables

[Asitha]

Description:
Need to explain MB tables in order to understand the Cluster mode vs the Stand-alone mode.
Need to explain with more details Slot-based message delivery with Cluster mode.

Original issue created by @fshomou in message-broker repo [1]
[1] https://github.com/wso2/message-broker/issues/170

Suggested Labels:
Documenation

Suggested Assignees:

Affected Product Version:
message-broker 3.2.0

OS, DB, other environment details and versions:

Steps to reproduce:

Related Issues:

[fshomou]

Thanks @Asitha.