Istio blogs should have a preview image

[kfaseela]

Currently, all our documentation when posted in social media do not have a preview image. Our documentation infra can be enhanced by updating the theme to pull an image from the document if one exists.

For eg: https://twitter.com/IstioMesh/status/1749523008077668542

[craigbox]

source of that (and all) pages includes:

<meta property="og:image" content="https://raw.githubusercontent.com/istio/istio.io/master/static/img/istio-social.svg">

That used to be enough.

https://developer.twitter.com/en/docs/twitter-for-websites/cards/guides/getting-started may help explain.

We need to work that out b efore adding support for ustom cards.

[craigbox]

(A better fix might be to start using the provided [opengraph.html](https://github.com/gohugoio/hugo/blob/master/tpl/tplimpl/embedded/templates/opengraph.html) and [twitter_cards.html](https://github.com/gohugoio/hugo/blob/master/tpl/tplimpl/embedded/templates/twitter_cards.html) templates.)