Meta tags for sharing in docs

[mostlyobvious]

Make shared links more precise with:

https://developers.facebook.com/docs/sharing/webmasters/ http://ogp.me https://developer.twitter.com/en/docs/tweets/optimize-with-cards/guides/getting-started.html https://github.com/rom-rb/rom-rb.org/blob/84d36d15cc9ac1fc7c993bb8cf437301909b5af0/source/layouts/_meta.slim https://github.com/rom-rb/rom-rb.org/blob/84d36d15cc9ac1fc7c993bb8cf437301909b5af0/config.rb#L96-L110