The guide mentions that the 'rails' user is needed, but doesn't
explicitly include the command to do so.

We have three or four different introduction sentences to the guides.
After this commit, we use the same one everywhere.

This reverts commit f5b9ed4f.

REASON: Translation work can not be done in the docrails repository

Fix small typo a link to bundler page in development_dependencies_install.md 

The purpose of this refactor is twofold:

  * Emphasize the Rails development box as
    the recommended way to setup a dev
    environment by taking out the rest of
    the instructions.

  * Lighten the contributing guide, this
    non-essential information takes a
    disproportionate amount of space now
    that we have the rails-dev-box.