Expand the intro docs

[markwalkom]

It'd be fantastic if we had an intro to the Elastic Stack and it's various use cases under the Overview page. I'm thinking of 3 high level areas we can start with;

• IT Analytics - System logs and metrics, security
• Business Analytics - App logs and metrics
• Search - Traditionally text only

Using generic use cases for each, we can then break the stack down and say "for X, install A", "don't install logstash for search" etc, etc.

[dedemorton]

As a first step, we should provide a high level overview of the stack and how the components work together.

[nrichers]

Subscribing.