add eleventy example This patch adds an Eleventy (https://www.11ty.dev/) sample manifest. Due to the nature of Node.js development, this example is somewhat more opinionated than the Hugo example: * A version of Node.js must be specified, so I chose the current LTS version. * The environment variable `CI` is necessary to avoid flooding the console with a fancy progress bar during `npm install`. * This example assumes the repository already contains a `package.json` file with `@11ty/eleventy` in its dependencies. I've also tweaked the style in the following ways: * De-mystify some of the workings of this manifest by linking to documentation where appropriate.