Discourse has this awesome feature of “oneboxes”. Here’s a quick example of a onebox that links to the official Discourse forum thread about oneboxes:
They work like this: Write a link in a line, alone. There should be nothing else in that line, and the link has to start with http:// or https://
Note that this only works for sites that have open graph tags enabled! So, it will work for popular sites, but probably not for most small sites.