Improve documentation structure

[pkosiec]

Overview

Currently, on docs.botkube.io I can spot the following problems:

• Open source and Cloud docs are mixed together
  • Example: Source config, which is self-hosted config only, and Prometheus source config which is Cloud-only
  • Maybe we should split the docs?
• We have separate Configuration and Usage sections for each plugin which requires user to jump between these two sections
  • it would make sense to merge them into a single doc, for example: Prometheus config + Prometheus Usage

Reason

The current documentation structure is confusing.