unfurl url should have a preview image

[0xpatrickdev]

We do not get the best unfurl preview for the https://docs.agoric.com url:

Fixing this, is straight forward. Seems we need to

• [ ] Determine what image we want to use - vitepress uses https://vitepress.dev/vitepress-og.jpg

Proposed (winged it with cmd+ctrl+shift+4, so maybe not centered):

• Add relevant meta tags. It seems to be at least -

  • <meta property="og:image" content="/images/agoric-og.png">

  • <meta property="twitter:card" content="/images/agoric-og.png">

  • [ ] site title

  • [ ] site description

  • optionally, keywords

  • reference:

  • https://web.dev/learn/html/metadata#description

  • https://web.dev/learn/html/metadata#open_graph