Add missing ckpt in config docs
# What does this PR do?

As discussed on Slack, I worked on the `Config` files to add missing information about checkpoints, or correct them.

- I tried to check the mentioned checkpoints are actually on the Hub
- also tried to make sure the checkpoints are for the target architecture
- I didn't verify the statement `Instantiating a configuration with the defaults will yield a similar configuration to that of the Speech2Text2 [mentioned checkpoint]`
   - in particular, the hyperparameters like `hidden_dim`, `num_layers` might be different
   - it says `similar`, so I think it is fine (..?)

@patrickvonplaten Could you take a look on the speech models?
@NielsRogge Could you take a look on the vision models?
_The documentation is not available anymore as the PR was closed or merged._<|||||>> Thanks for fixing all of those! It would be awesome to have some kind of quality script to check we don't introduce new faulty checkpoints.

Yes, I do have some (draft) check locally. I plan to add it in another PR (unless it's necessary to do so in this PR).<|||||>Thank you @NielsRogge I should try to use the correct names, as defined in `MODEL_NAMES_MAPPING`.<|||||>> Thanks a lot for this PR, awesome that this gets improved.
> 
> Left some comments, just for consistency, I would always use the template:
> 
> > "will yield a similar configuration of that of the - snake-cased model name -  [checkpoint name](link) architecture".

I will add this to the check I currently have (locally, but will push to another PR), thanks!<|||||>Merge now. Thanks for the review.

With this PR, all configs are good except the following (which are expected, since those composite models don't have full default config arguments - they rely on the encoder and decoder configs.)

- DecisionTransformerConfig
- VisionEncoderDecoderConfig
- VisionTextDualEncoderConfig
- CLIPConfig
- SpeechEncoderDecoderConfig
- EncoderDecoderConfig
- RagConfig

