[skip-ci] Some small improvements in the doc

[couet]

• doxygen automatically makes reference links to tutorials when then are simply named tutorial.C. In several places some tutorials were mentioned as $ROOTSYS/tutorials/path_to_the_tutorial/tutorial.C. This prevented doxygen to automatically make the link to the tutorial.C. This PR simplify $ROOTSYS/tutorials/path_to_the_tutorial/tutorial.C to tutorial.C to allow doxygen to make the link.
• remove \macro_image fromtree502_staff.C because the macro does not generate any image when run in batch.
• in Pythia8.cxx: remove the reference to pythia8.C which does not exist

[bellenot]

• in Pythia8.cxx: remove the reference to pythia8.C which does not exist

FYI, pythia8.C is now tutorials\evegen\evegen_000_pythia8.C