First commit to make some structure in doc/ directory.
- moved source/blender/python/doc -> doc/python_api
- moved source/gameengine/PyDoc/*.rst -> doc/python_api/rst
- modified accordingly sphinx_doc_gen.py and sphinx_doc_gen.sh
(later on I'll try alternative/ scripts by neXyon as promised :)
- source/gameengine/PyDoc/ is still there because contains epydoc stuff for the bge, will ask more and look into it later
