GitHub does not provide archiving.
If you want to consider a workaround, you can emulate archives locally by fully cloning the repository to a local machine. This will make you responsible for your own backups.
Issues can be exported using our API, while wiki pages can be exported by cloning the Git repository used for storing the wiki. You can get the URL for the wiki's Git repository by visiting the main wiki page and clicking on the "Clone URL" button.