Guide: Delete report specifications

They belong in the Help.
Explain how to use custom reports etc.

 * Guide: rename options link anchor
 * Help:
   * Remove note about this being a work in progress (since we're
     working in a separate branch now)
   * Remove listing of common report options (I've added a smaller
     listing to the Guide, and am planning to fully document all options
     for each report)
   * Add section on managing GnuCash custom reports