code: function(config, stream, extras, callback) {
      _.set(extras, "pageLocals.toc", placeHolder);
      _.set(extras, "layoutLocals.toc", placeHolder);
      callback(null, config, stream, extras);
    }
docs: When the files are loaded, we add a liquid local that simply replace {{ toc }}  with a string placeholder. This is needed because liquid runs before markdown conversion, and the TOC is generated after markdown conversion. So in a later hook, we generate the TOC and insert the TOC instead of the placeholder.