Migrate doc files to Markdown. (#24376)

* Rename index.mdx to index.md

* With saved modifs

* Address review comment

* Treat all files

* .mdx -> .md

* Remove special char

* Update utils/tests_fetcher.py

Co-authored-by: Lysandre Debut <lysandre.debut@reseau.eseo.fr>

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Co-authored-by: Lysandre Debut <lysandre.debut@reseau.eseo.fr>

docs/source/en/main_classes/text_generation.md

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+<!--Copyright 2022 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
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+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.
+
+⚠️ Note that this file is in Markdown but contain specific syntax for our doc-builder (similar to MDX) that may not be
+rendered properly in your Markdown viewer.
+
+-->
+
+# Generation
+
+Each framework has a generate method for text generation implemented in their respective `GenerationMixin` class:
+
+- PyTorch [`~generation.GenerationMixin.generate`] is implemented in [`~generation.GenerationMixin`].
+- TensorFlow [`~generation.TFGenerationMixin.generate`] is implemented in [`~generation.TFGenerationMixin`].
+- Flax/JAX [`~generation.FlaxGenerationMixin.generate`] is implemented in [`~generation.FlaxGenerationMixin`].
+
+Regardless of your framework of choice, you can parameterize the generate method with a [`~generation.GenerationConfig`]
+class instance. Please refer to this class for the complete list of generation parameters, which control the behavior
+of the generation method.
+
+To learn how to inspect a model's generation configuration, what are the defaults, how to change the parameters ad hoc,
+and how to create and save a customized generation configuration, refer to the
+[text generation strategies guide](../generation_strategies). The guide also explains how to use related features,
+like token streaming.
+
+## GenerationConfig
+
+[[autodoc]] generation.GenerationConfig
+	- from_pretrained
+	- from_model_config
+	- save_pretrained
+
+## GenerationMixin
+
+[[autodoc]] generation.GenerationMixin
+	- generate
+	- compute_transition_scores
+	- greedy_search
+	- sample
+	- beam_search
+	- beam_sample
+	- contrastive_search
+	- group_beam_search
+	- constrained_beam_search
+
+## TFGenerationMixin
+
+[[autodoc]] generation.TFGenerationMixin
+	- generate
+	- compute_transition_scores
+
+## FlaxGenerationMixin
+
+[[autodoc]] generation.FlaxGenerationMixin
+	- generate