Add social media cards

[hturner]

Is your feature request related to a problem? Please describe.

Currently there is no image when sharing https://contributor.r-project.org/rdevguide/ on Twitter, Slack, etc

Describe the solution you'd like

A card with an image should be generated by the social media platform when sharing https://contributor.r-project.org/rdevguide/. The image should be the same as the cover image.

Describe alternatives you've considered

N/A

Additional context

The required HTML metadata should be created automatically if we set the correct metadata, but we should validate the results: https://bookdown.org/yihui/bookdown/features-for-html-publishing.html#metadata-for-sharing

[hturner]

This works now we have the correct metadata, confirmed here: https://www.opengraph.xyz/url/https%3A%2F%2Fcontributor.r-project.org%2Frdevguide