Update directory structure of documentation models.
    
    - Previously, style sheets and xsl files used to transform
      documentation source files were stored in the root directory of
      tcar-models-docs branch. This organization is inappropriate when the
      tcar-models-docs branch is merged into tcar branch. In order to
      provide a better organization, this update creates the
      Models/Documentation/Css and Models/Documentation/Xsl directories
      and relocates css and xsl files inside them.