There's no built in functionality to do this. There may be a plugin but I don't know of one.
Idea: preview the links topics
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If you post a link Github - to get the teaser:
https://github.com/NodeBB/NodeBB/issues/3593
How do you like the idea to do the same, only with reference to the topic?
Example:
https://community.nodebb.org/topic/180/who-is-using-nodebb -
If you're after a general preview of a link's content, you'd want to try embedly (I can't remember off the top of my head if there's an embedly plugin for nodebb)
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Don't think there is. But the api could be used to create one.