Add social image preview to knative.dev

[samodell]

Copied from the Docs repo: https://github.com/knative/docs/issues/1696

From @steren :

When I share https://knative.dev/ on Twitter, no image thumbnail appears in the twitter card. You can test this at https://cards-dev.twitter.com/validator

How to fix:

Add the appropriate tags so that Twitter and other social networks pick up the Knative logo as thumbnail as well as a nice summary