Port markdown filter: Link rewrites

Implement link rewriting. We need to be able to do two things:

1. Convert links to .md files to their HTML counterparts. This is how we do this in Nanoc: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/lib/filters/markdown_to_html_ext.rb?ref_type=heads
2. Allow for changing the base URL as part of the build. See if we can replace normalize-links.sh with something at build time. (this can be controlled by Hugo configuration files! I added an issue for that for later here: #57 (closed))

In Hugo this will likely be a markdown render hook. See https://www.veriphor.com/articles/link-and-image-render-hooks/ for ideas.