Passing raw HTML tags through the markdown processor

[Comments from Discord](https://discord.com/channels/872106850928164874/872106850928164878/967013217358462986):

> [Graf] I have several raw HTML tags in my Markdown (.md) files. How do I return them as-is to be shown when I call post.html? They are currently removed from it
>
> [schickling] I think there's some rehype/remark plugin that can do this.
>
> [Graf] I attempted to use rehype-raw but no dice.. ended up just using the mdx format instead to preserve the tags

Let's solve this and then add the solution to the docs.