Intermittent 429 status code result for og images of project cards

There seems to be some rate limiting on the opengraph endpoint that is currently used to reference the image source.

Instead of the open graph images, let's just use a generic static placeholder image that is served as part of our next.js app.

The social preview images don't have the same problem, so these should remain. However, it would make great sense to also copy these images to the static build output of the next.js app, so that users of the app don't need to connect to the github host for social previews when visiting the site.