Fix 9918
# What does this PR do?

This PR addresses the problem shown in #9918 by:
- adding the documentation of the `encode` method to the `PreTrainedTokenizer` and `PreTrainedTokenizerFast` (it is in all their subclasses already)
- adding the "What are input IDs" link where missing in some models docstrings.

In passing, I uncovered a failure of the doc styling script on DPR, so this PR also fixes that.

Fixes #9918