Add a “pages” submodule to github

[sellout]

Publishing to pages is a lot of rendundant code. The bulk of the workflow should be produced automatically (along with some github settings) and then projects only need to set whether it should be populated from a branch or actions, and (if actions), the with arguments to pass to nix-develop-command.

E.g., the config should look like one of the following:

services.github.pages = {
  enable = true;
  branch = "docs";
};

or

services.github.pages = {
  enable = true;
  action = {
    command = ''
      idris --mkdoc iaia.ipkg
      ## We copy here to fix the permissions from the Nix symlinks
      cp -r ./iaia_doc ./_site
      chmod --recursive +rwx ./_site
    '';
    devshell = "iaia";
  };
};