docs: Fix links in markdown content differently

pandoc insists on using the xlink namespace for hrefs, and the namespace setup doesn't carry over xi:includes. My first fix was to tell pandoc to generate standalone docbook documents, which makes it insert the xlink namespace. But it also makes it wrap all sections and chapters in articles, and that messes up our toc structure.

So, patch things up differently by stripping the xlink: from hrefs via regex.

Yay for XML!