Make it possible to generate the API docs for only newer versions

[ntamas]

Simply adding a new version for which we want to generate API documentation in the Makefile has no effect; running make will not generate the API documentation if it has been generated earlier for a smaller set of versions because all the "stamp" files are unversioned. E.g., after adding a new Python version, I cannot simply run make python to update the Python docs. Running make clean_python && make python does the trick; however, it removes the entire Python build folder and all the generated Python docs so everything will be re-built from scratch. This will become a bottleneck as soon as we start having more versions in the makefile.

The ideal solution would be for make python to recognize that a new version was added and to build the documentation for that version only, leaving all previously generated versions alone.