Development environment setup instructions refactored

  * New "Fuel Development Environment" section
  * Setup instructions from develop/nailgun/development and develop/iso
    and moved into the new section
  * Nailgun instructions are split into multiple sections for backend
    unit tests, UI tests, fake mode,
  * Added instructions on how to run Astute unit tests
  * Added instructions on how to build the documentation
  * Dependency installation is biased to use distribution packages where
    available and minimize use of pip, gem, and npm
  * Multi-line paragraphs used for better source readability