docs: improve documentation for markdown-it & eleventy integration

[christianvoigt]

Currently the docs use "import" syntax instead of "require" even though this will not work with eleventy. Here is a working integration in .eleventy.js:

const mdi = require("markdown-it");
const argdownConfig = {logLevel: "verbose"};
const createArgdownPlugin = require("@argdown/markdown-it-plugin").default;
const markdownItArgdown = createArgdownPlugin(argdownConfig);
const mdiInstance = mdi().use(markdownItArgdown);
module.exports = function(eleventyConfig) {
    eleventyConfig.setTemplateFormats([
        "html",
      "md",
      "css" // css is not yet a recognized template extension in Eleventy
    ]);
    eleventyConfig.setLibrary("md", mdiInstance);
  };