How to register components with the MDX renderer?

[milgner]

Description of issue

I want my MDX file to render a custom template component but it's not clear where I need to register it in order to make it available. Currently it says:

Component ADRList was not imported, exported, or provided by MDXProvider as global scope

Usage example

The project in question uses a multi-level structure which cannot (and should not) be represented through the sidebar.
Instead, I'd like to put documents into the directory and have the index file create a TOC for all other MDX files in the same directory. As such, I created a Gatsby template component that could be included in the MDX, but I don't know where to register it.