better links to GEO and API tutorials (cf. #795)

doc/texinfo/gmsh.texi

tutorial/README.txt

tutorial/c++/t10.cpp

tutorial/c++/t13.cpp

tutorial/c++/t17.cpp

tutorial/c++/t5.cpp

tutorial/c++/t7.cpp

tutorial/c++/x1.cpp

tutorial/python/t10.py

tutorial/python/t13.py

tutorial/python/t17.py

tutorial/python/t5.py

tutorial/python/t7.py

tutorial/python/x1.py

tutorial/t10.geo

tutorial/t11.geo

tutorial/t13.geo

tutorial/t17.geo

tutorial/t5.geo

tutorial/t7.geo