Update repository documentation manual:
    
        - Remove information related to TreeFlower background construction
          outside the version schema of artistic motifs. Also, this
          information mention manual consturction of .ppm and .hex
          palettes which is no longer needed since that process has been
          automated already.
    
        - Instead of having the same description for each artistic motif
          directory structure, we concentrate documentation inside design
          models directory structure and make redirection links from
          artistic motif documentation entries to design models
          documentation entries. An icon to illustrate redirection could
          be nice when building redirection specific admonitions.