Remove trunk/ from paths in Docbook documentation models.
    
    Previous to this commit, paths passed to Docbook external references
    inside Docbook documentation models pointed to files under a trunk/
    directory.
    
    In this commit, I remove the trunk/ part from paths. It is no longer
    needed when we use a Git repository layout. To remove the trunk parth
    from paths I used the following commands:
    
     - sed -i -r 's!trunk/!!g' $(find Documentation/Models/Docbook -name '*.docbook')
     - sed -i -r 's!trunk/!!g' $(find Documentation/Models/Docbook -name '*.ent')
    
    The change committed here may introduce incoherences to contents
    inside documentation manuals written in Docbook format that need to be
    fixed.
    
    In addition to change committed here, the following Docbook entities
    were also added along with it to Tcar-ug documentation manual:
    
     - TCL: The CentOS Logo
     - TCS: The CentOS Symbol
     - TCT: The CentOS Type
     - TCB: The CentOS Brand