add og:image attribute (#3003)
add og:image attribute (#3003)

Summary:
This should set the `og:image` meta property that determines the scrape
preview.
Pull Request resolved: https://github.com/facebook/relay/pull/3003

Test Plan:
Docusaurus docs here:
https://docusaurus.io/docs/en/site-config#ogimage-string

Should show the Relay logo after the website deploys in this tool: https://developers.facebook.com/tools/debug/?q=relay.dev

Reviewed By: jstejada

Differential Revision: D19448289

Pulled By: kassens

fbshipit-source-id: 26a9b69e4ce3f1b91e9bc59a11e3c2822733cca7

kassens/relay,facebook/relay,facebook/relay,wincent/relay,xuorig/relay,kassens/relay,xuorig/relay,facebook/relay,facebook/relay,iamchenxin/relay,iamchenxin/relay,yungsters/relay,josephsavona/relay,josephsavona/relay,facebook/relay,wincent/relay,kassens/relay,atxwebs/relay,kassens/relay,cpojer/relay,voideanvalue/relay,wincent/relay,cpojer/relay,cpojer/relay,wincent/relay,cpojer/relay,josephsavona/relay,xuorig/relay,iamchenxin/relay,voideanvalue/relay,voideanvalue/relay,wincent/relay,facebook/relay,xuorig/relay,voideanvalue/relay,atxwebs/relay,kassens/relay,xuorig/relay,kassens/relay,wincent/relay,iamchenxin/relay,voideanvalue/relay,xuorig/relay,yungsters/relay,yungsters/relay,voideanvalue/relay,atxwebs/relay,atxwebs/relay