Twitter card image has to be fully qualified

[hybridcattt]

I just spent some time debugging why an image specified in twitter:image tag wasn't showing up on Twitter.
Turns out, the url has to be full qualified, it won't work if the url is local, such as /assets/blabla.jpg.

Here's the official commentary from Twitter staff: https://twittercommunity.com/t/twitter-card-image-is-not-showing/155184/4

I'm happy to put up a PR, I'm just wondering - is it something we should fix on the generation level, or just mention in the documentation?

[mattr-]

Thanks for writing up this issue! I think this is something we should definitely fix on the generation level. If you can post a PR to fix it, that would be lovely! ❤️

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