Sometimes problems fix themselves if you walk away for awhile and reboot your computer twice.
I followed this glossy website (https://pages.github.com/) and then the more nerdy OTHER github website (https://help.github.com/categories/20/articles). See specifically, https://help.github.com/articles/creating-project-pages-manually.
There were a number of problems with "git commit -a -m "First pages commit" but eventually they went away. Like after about an hour or more. Sorry, I should be polite. Now, no more 404s.