Relative linking in Hugo
Hugo doesn’t currently support relative links to files.
Instead, it expects you to link to other pages using the ref and relref shortcodes (or the similarly named functions from within templates).
It appears this was originally implemented in 2015 but less than two years later deprecated in 0.19 and removed in 0.26.
Markdown Render Hooks appear to provide a workaround.
This came up with obsidian-export, where I provide an example.