RFC: social cards

Issue for idea already expressed here: https://github.com/facebook/docusaurus/issues/2267#issuecomment-639597335

Dynamic Social Cards

You might have noticed that some people / companies have an automated way to generate social cards that adapt to the underlying content (generating an image from query string params)

I think this would cool if by default, all Docusaurus docs/blogs would generate custom social cards, if the user did not define any custom social image.

This way, all Docusaurus links pasted to a social media would have a beautiful image.

By default we could even include a discrete Docusaurus + Slash logo for growth hacking

Vercel has a nice project to do this easily: https://github.com/vercel/og-image

We could fork this project, host our own variant (can be free on Vercel), and use it for D2 website, + allow other sites to use it as well.

Curretly, Docusaurus can have a default social card, but it's static and does not adapt to the content::

Here's some examples of dynamic/automated social cards: