Add a man page

Currently, one can get the basic information from autogen itself by running it without any arguments or with the -h flag, but it would also be useful to be able to run man autogen to get all the information.

Note: it might conflict with, e.g., https://www.gnu.org/software/autogen/manual/autogen.html if we were to publish to the standard man page location with the name autogen, so we may need to choose a different name, or hold off on this for now.