[Doctest] Add configuartion_longformer.py
# What does this PR do?
Fixes # 19487

Add configuration_longformer.py to utils/documentation_tests.txt for doctest.

Based on issue https://github.com/huggingface/transformers/issues/19487

@sgugger could you take a look at it?
Thanks :)
Hi @AShreyam

This PR is not ready to merge. It contains other changes that is not for this config doctest sprint.
<|||||>Hi @AShreyam

This PR is not ready to merge. It contains other changes that is not for this config doctest sprint.
<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.