documentation_tests.txt - sort filenames alphabetically
# What does this PR do?

Reorganises the file names listed in `documentation_tests.txt` so that they are in alphabetical order. This is to address two things: 
* Make it obvious where to add new files
* Make it easier to spot if certain files are missing. For example, I didn't notice until recently that modleing_imagegpt.py wasn't included (its config was). 


## 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?

_The documentation is not available anymore as the PR was closed or merged._<|||||>@ydshieh I added a quick check in `utils/check_doctest_list.py` - let me know what you think :)<|||||>Yes! Eventually, we can provide a fix option to modify the file to sort and deduplicate the lines. But again, the PR itself is complete and can be merged already.