Improves guides by focusing on the tasks developers need to get done.

:bomb: This is a work in progress. Do not merge yet. 
(I'm doing this on top of the 2-1-x branch, hence the need for rebase)

The Integration section should support this two stories:
 - As a Storefront Developer, I want to connect my Storefront to The Hub.
 - As a Service Developer, I want to develop an endpoint to connect my Service to The Hub.

Changes:
 - [x] Integration index has buttons: "Connect your storefront" and "Develop an endpoint"
 - [x] Spree Commerce Hub > Configuration promoted to Getting Started. (with it's own index).
 - [x] Messages moved to Spree Commerce Hub > Message Types (after Message Basics).

Sections:
- Spree Commerce Hub (a documentation of how things work)
- Getting Started Guide (to connect your storefront)
- Tutorials (for developing an endpoint to integrate your service)
- Customization (developing endpoints for your own storefront)
- Supported Integrations (working integration, Mandrill, Quickbooks, submit your own button)
 
Ideas:
 - [ ] The header image should contain the storefront to better reflect the conceptual model of the integration. Something like: ![header image](https://www.evernote.com/shard/s3/sh/6507f273-10ee-4982-82bf-6261c8efb804/878afd1060db693987b89a2f41c7e88e/res/ea0f7239-7a0a-40d3-8291-764f2fb608e8/skitch.png?resizeSmall&width=500)