Use screenshot service to generate card image for documentation pages

[mojavelinux]

To provide a more useful image when sharing a documentation page on social media, the image should show a preview of the page using a screenshot service.

Eleventy uses a screenshot service to make a screenshot of the documentation page as the preview image. See https://opengraph.xyz/url/https%3A%2F%2Fwww.11ty.dev%2Fdocs%2Flayouts%2F for an example and the following post for how it's implemented: https://zachleat.com/web/screenshots