Improve open graph API metadata

[turbaszek]

It would be nice to improve open graph metadata which are used to render previews of our website in socials medias.

I used this page to check how it looks currently: https://www.opengraph.xyz/url/https:%2F%2Fairflow.apache.org/

and it seems that there's no image. I can fix it but not sure where the open graph template should be added: https://gohugo.io/templates/internal/#open-graph