Feature/fix slow test in mluke
# What does this PR do?

Fix slow tests in MLuke/Luke (https://github.com/huggingface/transformers/pull/14690).

* Replace tokenizers to dummy tokenizers in `MLukeTokenizerTest/LukeTokenizerTest`.
* Add a missing entry in toctree.

## Who can review?
@sgugger @NielsRogge

Merging this :)