Transform relative paths to URL slugs on build

[robinmetral]

Something I really want is to use relative paths for page links. Benefits:

• navigate the site directly in the IDE
• get autocomplete from the IDE
• ...which helps avoid creating links when authoring

<!-- in /posts/2.md -->
A link to [the about page](../about.md) and a link to [another post](./1.md).

Currently, this would obviously break the build. We'd need to go through all links on build and transform them. Good news is that this is pretty easy using the hast that unified gives us. If using this in html pages, we need #42 first, to make sure html pages go through a hast step.

Eleventy does something like this in a plugin: https://www.11ty.dev/docs/plugins/inputpath-to-url/