Add variable definitions to `manual.texinfo' file used to create
    manual's main definition file. 
    
        - These variables are mainly used to store information like
          official names and URLs repeated several times inside the
          manual. Instead of changing the same information time after
          time along the manual, we create a variable name for each one of
          them and use the variable value in the manual whenever the value
          set needs to be expanded.
    
        - The advantage of this configuration is when one of those
          repeated values needs to be changed. There is no need to change
          them all along the manual, but their definition only.  It
          definitly helps to maintain content inside the manual
          consistent.