Install KWSys Copyright.txt as documentation

This commit adds KWSys configuration option KWSYS_INSTALL_DOC_DIR to
specify the directory for installation of documentation.  We use it to
put the KWSys Copyright.txt file at the location

  ${KWSYS_INSTALL_DOC_DIR}/${KWSYS_NAMESPACE}/Copyright.txt

in the project installation tree.  This helps containing projects meet
the license requirement to distribute the copyright and license with
binary forms.