Add CI/Sphinx plugin to verify that "supported targets" is up to date

[Schamper]

From #84:

If we make it manual, we should add a CI test/Sphinx extension that throws a warning if there are more modules available in e.g. dissect.target.loaders.\*\* then there are in the table.

There are a few we intentionally skip, so those should be accounted for in the test script.

Probably a Sphinx plugin is easiest?