Cross reference CXX_STANDARD and CXX_EXTENSIONS
Update the documentation for C/CXX_STANDARD
(https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD.html) to include a cross reference to the C/CXX_EXTENSIONS
page.
Update the documentation for C/CXX_STANDARD
(https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD.html) to include a cross reference to the C/CXX_EXTENSIONS
page.