Simplify/automate the addition of SEO/open graph meta tags

[imphil]

Adding SEO-related meta tags (OpenGraph tags, twitter cards, etc.) is currently a bit inconvenient.
Simplify that (e.g., take the first picture by default, make URLs absolute automatically, etc.)

See also

• https://content.nuxt.com/composables/use-content-head
• useContentHead should add og:image including base url. nuxt/content#2373