Rendering code replaces @xyz with github link in URLs, breaking link targets

[joshtriplett]

If you write something like [this document](https://hackmd.org/@xyz/document), the rendering code will replace @xyz with a github link, breaking the URL in the hackmd link.

This replacement needs to be markdown-aware, and shouldn't happen in (at least) code blocks and link targets. If that's not easily possible, then we should disable the username-replacement logic until it can be fixed.

#203 was a PR that resulted from having to work around this breakage.

[nikomatsakis]

Yeah, it's currently just some simple regexs. It might be worth breaking out a real markdown parser.

[nikomatsakis]

It seems like comrak can be easily made to do what is needed...

https://github.com/kivikakk/comrak?tab=readme-ov-file#usage