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/model_doc/retribert.md

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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.
+
+⚠️ 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.
+
+-->
+
+# RetriBERT
+
+## Overview
+
+The RetriBERT model was proposed in the blog post [Explain Anything Like I'm Five: A Model for Open Domain Long Form
+Question Answering](https://yjernite.github.io/lfqa.html). RetriBERT is a small model that uses either a single or
+pair of BERT encoders with lower-dimension projection for dense semantic indexing of text.
+
+This model was contributed by [yjernite](https://huggingface.co/yjernite). Code to train and use the model can be
+found [here](https://github.com/huggingface/transformers/tree/main/examples/research-projects/distillation).
+
+
+## RetriBertConfig
+
+[[autodoc]] RetriBertConfig
+
+## RetriBertTokenizer
+
+[[autodoc]] RetriBertTokenizer
+
+## RetriBertTokenizerFast
+
+[[autodoc]] RetriBertTokenizerFast
+
+## RetriBertModel
+
+[[autodoc]] RetriBertModel
+    - forward