Add a check on config classes docstring checkpoints
# What does this PR do?

A follow-up for #16900: add a test to make sure all config classes have at least one valid checkpoint (unless explicitly specified to ignore).

By `valid`, it only means the format is valid, i.e. of the form `[XXX](https://huggingface.co/XXX)` with `XXX` being any string.
A more strict verification could be implemented by trying to load the config. But maybe it is a bit too much?

Also fix 2 more config classes without valid checkpoint.
_The documentation is not available anymore as the PR was closed or merged._