

TITLE
How to fine-tune NLP tasks docs

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This PR focuses on adding guides for how-to fine-tune models for NLP tasks. Main additions:

- Change the TOC structure to include a nested section in the how-to guides for fine-tuning for NLP tasks.
- Separate each task in `custom_datasets` into their own pages. This will make it easier for users to find and read about a specific one. Also, when you click on any of the subsections that share the same name (`Preprocess`, `Fine-tune with Trainer API`, and `Fine-tune with TensorFlow`), you are automatically redirected to the sequence classification section even if you wanted to look at the question answering section.
- Add a guide for causal/masked language modeling.

To do:
- [x] Add guides for multiple choice, translation and summarization.