Add information about custom roles in Admin namespace.

- Explain to user how to authorize specific admin controllers with
CanCan

- Explain to user the importance of "model_class" when defining custom
controllers in the Admin namespace.

Fixes #143
[security] Add further information about providing patches for security issues
[dev] make documentation about master variants clearer
Fixed navbar links for API guide
Update order_messages.md

use spree naming convention for adjustments
Updated JSON for shipment messages
Added image to configuration guide showing connector was installed successfully.
Added 'status' to shipment_messages
Updated the reference JSON for order messages
Applied parse_info_boxes to /api and /user sections.
Add details from CONTRIBUTING.md in repo to Contrib guide, move 'How we Prioritize Issues' section under 'Issues' heading
fix install command
Update order_messages.md

slightly modify payments for import
added purchase order message
Fix shipment:ready
Updated order and shipment message formats
Example for updating order ship method through api

Fixes #146
Fixed a bunch of bookmark naming issues in the dev shipments guide.
Mention requirement of shipping_category_id on Product.
Add shipping category id in example.