Fix promotion chain config example

Fixes #84
Added stock locations api docs
Added stock items api docs
Added stock movements api docs
Significant updates to the order checkouts guide
Update api shipment docs to reflect new functionality
Add mention of nested_taxons_path to products page
Authentication Guide
Document the process of updating an order
Add mention of Git setup + cover more of application setup in Heroku guide
Integrator rewrite WIP
More integrator guides
placeholders for required CONSTs
WIP new shipments guide
Add stock management section to inventory guide
More progress on shipping guide
Small improvements to readability - matches the wording in the spree_fancy README edited in commit https://github.com/j1mr10rd4n/spree_fancy/commit/fcd5905084ed49ccc185ec2f80d1f7d74fc55379
Added placeholder for decorators tutorial page
More WIP integrator changes
Guide Migrations