If you want to preview markdown, you need a markdown compiler. There is no markdown compiler written in Elisp, so no, it cannot be integrated into Doom Emacs. You could, however, install a markdown viewer plugin in your browser, like Markdown Viewer – Get this Extension for 🦊 Firefox (en-US), to get low-effort markdown previews.
This is the reason. Everything easy to use with Doom Emacs. Searching files or words don’t need to install third party library. So I think if markdown preview can easily use will be more awesome.
Thank you for your suggestion about the browser plugin. It looks good to use.