I’m not following you.
A Discourse plugin that incorporates markdown-it does not exist, of course you can not install plugins written for other platforms.
If you really need the markdown-it extra-features (which as Jeff said is not the standard markdown we use on Discourse) you should write a plugin, or open a topic on Marketplace to find a dev to pay that write it for you.
I can not understand what you mean. Any official and unofficial plugin, if open to the public of course, can be installed (at your own risk) on Discourse if your instance is self-hosted.
The “plugin” you linked is not a plugin, it’s a search with 0 results for me…
If you search for “discourse” you can find something, but nothing related to markdown-it.
I do not know if there is anything available only for enterprise level, in this case take a screenshot to let us see.