Enable doc in Spanish (#16518)

* Reorganize doc for multilingual support

* Fix style

* Style

* Toc trees

* Adapt templates

docs/source/en/model_doc/mt5.mdx

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+<!--Copyright 2020 The HuggingFace Team. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+the License. You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+specific language governing permissions and limitations under the License.
+-->
+
+# mT5
+
+## Overview
+
+The mT5 model was presented in [mT5: A massively multilingual pre-trained text-to-text transformer](https://arxiv.org/abs/2010.11934) by Linting Xue, Noah Constant, Adam Roberts, Mihir Kale, Rami Al-Rfou, Aditya
+Siddhant, Aditya Barua, Colin Raffel.
+
+The abstract from the paper is the following:
+
+*The recent "Text-to-Text Transfer Transformer" (T5) leveraged a unified text-to-text format and scale to attain
+state-of-the-art results on a wide variety of English-language NLP tasks. In this paper, we introduce mT5, a
+multilingual variant of T5 that was pre-trained on a new Common Crawl-based dataset covering 101 languages. We detail
+the design and modified training of mT5 and demonstrate its state-of-the-art performance on many multilingual
+benchmarks. We also describe a simple technique to prevent "accidental translation" in the zero-shot setting, where a
+generative model chooses to (partially) translate its prediction into the wrong language. All of the code and model
+checkpoints used in this work are publicly available.*
+
+Note: mT5 was only pre-trained on [mC4](https://huggingface.co/datasets/mc4) excluding any supervised training.
+Therefore, this model has to be fine-tuned before it is useable on a downstream task, unlike the original T5 model.
+Since mT5 was pre-trained unsupervisedly, there's no real advantage to using a task prefix during single-task
+fine-tuning. If you are doing multi-task fine-tuning, you should use a prefix.
+
+Google has released the following variants:
+
+- [google/mt5-small](https://huggingface.co/google/mt5-small)
+
+- [google/mt5-base](https://huggingface.co/google/mt5-base)
+
+- [google/mt5-large](https://huggingface.co/google/mt5-large)
+
+- [google/mt5-xl](https://huggingface.co/google/mt5-xl)
+
+- [google/mt5-xxl](https://huggingface.co/google/mt5-xxl).
+
+This model was contributed by [patrickvonplaten](https://huggingface.co/patrickvonplaten). The original code can be
+found [here](https://github.com/google-research/multilingual-t5).
+
+## MT5Config
+
+[[autodoc]] MT5Config
+
+## MT5Tokenizer
+
+[[autodoc]] MT5Tokenizer
+
+See [`T5Tokenizer`] for all details.
+
+
+## MT5TokenizerFast
+
+[[autodoc]] MT5TokenizerFast
+
+See [`T5TokenizerFast`] for all details.
+
+
+## MT5Model
+
+[[autodoc]] MT5Model
+
+## MT5ForConditionalGeneration
+
+[[autodoc]] MT5ForConditionalGeneration
+
+## MT5EncoderModel
+
+[[autodoc]] MT5EncoderModel
+
+## TFMT5Model
+
+[[autodoc]] TFMT5Model
+
+## TFMT5ForConditionalGeneration
+
+[[autodoc]] TFMT5ForConditionalGeneration
+
+## TFMT5EncoderModel
+
+[[autodoc]] TFMT5EncoderModel
+
+## FlaxMT5Model
+
+[[autodoc]] FlaxMT5Model
+
+## FlaxMT5ForConditionalGeneration
+
+[[autodoc]] FlaxMT5ForConditionalGeneration