Implement a module for highlighting related content

In https://github.com/GoogleChrome/web.dev/pull/7775 I added some text linking to the new Learn PWA course.

It would be generally useful to have some kind of module we can add to relevant posts to cross-link to other useful stuff. I'm thinking of either a card below the navigation or something we can insert in a good place in the content. Given a lot of traffic comes to individual articles via search this would give us a way, for example, to signpost people to Learn PWA from each individual PWA article.

Content would be:

- Headline
- Short text
- Link
- Optional image and alt.

Ideally we could create these as data, so we can then just add the relevant module with a shortcode, rather than recreating it each time.

I'm creating this request on web.dev but it would also be useful on d.c.c.