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.

The issue names no files, tests, or entry points. Start by reproducing how raw HTML is handled in the Markdown processor and investigate the mentioned rehype-raw and remark-plugin approaches. Done means raw HTML tags are preserved in post.html and the working solution is added to the documentation.

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