Expand the intro docs

It'd be fantastic if we had an intro to the Elastic Stack and it's various use cases under the [Overview page](https://www.elastic.co/guide/en/elastic-stack/current/elastic-stack.html). 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.