Add the language name to the translated GGI handbook PDF's file name
Currently all files are named ggi_handbook.pdf in each handbook/translations/<lang_code> subfolder. Having the being a part of the file name makes it easier to know about the content without opening the file and to have all translated handbook in the same folder without the need to rename or the risk of overwriting. However, the English original and reference should not be renamed to show its special position.