Accessing "invalid" URLs (the path doesn't match what we expect --eg. the version is not in the URL) makes addons to not load.

https://docs.readthedocs.com/platform/notfound/

In that URL, notfound is detected as the version slug --which doesn't exist-- and the call to https://docs.readthedocs.com/_/addons/?client-version=0.43.0&api-version=1&project-slug=docs&version-slug=notfound fails with 404.

However, the call to https://docs.readthedocs.com/_/addons/?client-version=0.43.0&api-version=1&url=https://docs.readthedocs.com/platform/notfound/ (note that we are using url= instead of project/version slug) and it works fine with 200 and returns versions.current: null 👍🏼

Note the version slug comes from X-RTD-Version: notfound -- so, we may want to check the version exists before returning this header. However, I'm not sure what's the correct way to make this work.