Hello Ethan,
I don't think I have seen an external developer do this before, and as far as I know there is nothing built into the SDK to build documentation off of your SDK extensions. However, the documentation for the SDK is built using Sphinx. So you are welcome to use the code in the sdk repository and learn how Q2 builds those docs to build your own. https://code.q2developer.com/Q2Antilles/q2-sdk Note if you want this site always up and running somewhere you would have to host it. Might be easier to build and run this documentation site locally or somewhere in your organization. Note, Gitlab does have nice support for Readme docs and other markdown files which uses markdown and can do code references and things of the like. So if you haven't done that yet I would recommend starting there.
-Trey