Add Fuel development quick-start guide

This update adds a quick-start guide to
the fuel development documentation, and
fixes a broken link in the Nailgun
development guide.

Change-Id: I51c6add75679f1a9285f4986a07fe516ebead2ec
Closes-Bug: #1332655
Implements: blueprint fuel-web-docs-dev-env-restructure
Merge "Add Fuel development quick-start guide"