Sphinx inventory is missing for UJS

[Julian]

This link: https://json-schema.org/understanding-json-schema/objects.inv

on the old website was a Sphinx inventory for Understanding JSON Schema.

It now appears to be a 404.

I know we don't plan on sticking with Sphinx, or perhaps have already switched away from it as part of the website move and I simply didn't know the 2 were happening together -- though regardless, even if we use some other documentation tool, it would be nice to at least consider continuing to provide this file or an equivalent. Its purpose is pretty "universal" in nature -- it's meant to help automated tooling understand where links should go within the site, so other documentation systems likely still could be coerced into producing one.

(This issue essentially breaks sphinx-json-schema-spec and transitively python-jsonschema's documentation, though if it turns out we've now officially moved over to something else obviously I'll rework those projects.)