Add opengraph metadata, so url previews are more meaningful

[mweinelt]

Describe the glitch
The url preview in Matrix, and possibly Twitter, sites that rely on open graph data for more meaningful previews, could use some work, so that we don't accidentally have previews like this one, where a sponsor is shown as the primary graphic for the site.

Screenshots

Additional context
https://ogp.me/

This is what GitHub does:

    <meta property="og:image" content="https://opengraph.githubassets.com/1ece67598c5b8e2ed59148e399c3735d5ef7acedbfb4555df6d38bf87730af6b/NixOS/nixos-homepage/issues/711" /><meta property="og:image:alt" content="Describe the glitch The url preview in Matrix, and possibly Twitter, sites that rely on open graph data for more meaningful previews, could use some work, so that we don&amp;#39;t accidentally have pre..." /><meta property="og:image:width" content="1200" /><meta property="og:image:height" content="600" /><meta property="og:site_name" content="GitHub" /><meta property="og:type" content="object" /><meta property="og:title" content="Add opengraph metadata, so url previews are more meaningful · Issue #711 · NixOS/nixos-homepage" /><meta property="og:url" content="https://github.com/NixOS/nixos-homepage/issues/711" /><meta property="og:description" content="Describe the glitch The url preview in Matrix, and possibly Twitter, sites that rely on open graph data for more meaningful previews, could use some work, so that we don&amp;#39;t accidentally have pre..." />

[andir]

Related #712