[Doctest] - Fixing doctest `configuration_pegasus_x.py`
# What does this PR do?
Fixes #19487 
Added (with random weights) in `configuration_pegasus_x.py`.
Added `configuration_pegasus_x.py` in `documentation_tests.txt`.

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?
@ydshieh 

Hi @mukesh663 There seems to be some CI issue. Could you push again with an empty commit?

You can do `git commit --allow-empty` then push :pray.<|||||>Hi, @mukesh663 I haven't merge your PR yet ...but you already closed the PR and delete the branch 😢 <|||||>Are you able to restore the branch ?<|||||>> Are you able to restore the branch ?

@ydshieh Sorry for the mishap. I have restored the branch.
<|||||>Merged, thank you again!<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_19725). All of your documentation changes will be reflected on that endpoint.