various fixes and (minor) improvements to documentation

GaudiKernel/GaudiKernel/AttribStringParser.h

GaudiKernel/GaudiKernel/CArrayAsProperty.h

GaudiRelease/doc/DoxyCommon.cfg

GaudiRelease/doc/DoxyFile.cfg

GaudiRelease/doc/DoxyTags.cfg

GaudiRelease/doc/Doxyfooter.html

GaudiRelease/doc/DoxygenMain.doc

GaudiRelease/doc/Doxyheader.html

GaudiRelease/doc/dottool.cfg

GaudiRelease/doc/doxygen/CMakeLists.txt

GaudiRelease/doc/doxygen/Doxyfile.in

GaudiRelease/doc/doxygen/get_cppreference_tags.cmake

GaudiRelease/doc/doxygen/issue_tracker_links.js

GaudiRelease/doc/manual/Manual.md

GaudiRelease/prepare_gaudi_release.py