Add OpenGraph / embed

[danieldegroot2]

There currently seems to be no embed for uMap (at least not when posted on Discord). This can be used to give a comprehensive preview of the website when linked on i.e. social media.

[davidbgk]

The minimal pertinent informations for OpenGraph:

<meta property="og:url" content="https://adactio.com/journal/9881">
<meta property="og:title" content="Metadata markup">
<meta property="og:description" content="So many standards to choose from.">
<meta property="og:image" content="https://adactio.com/icon.png">

See https://web.archive.org/web/20201204172748/https://www.broken-links.com/2015/12/01/little-less-metacrap/