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/internal/image_processing_utils.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
+
+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.
+
+-->
+
+# Utilities for Image Processors
+
+This page lists all the utility functions used by the image processors, mainly the functional
+transformations used to process the images.
+
+Most of those are only useful if you are studying the code of the image processors in the library.
+
+## Image Transformations
+
+[[autodoc]] image_transforms.center_crop
+
+[[autodoc]] image_transforms.center_to_corners_format
+
+[[autodoc]] image_transforms.corners_to_center_format
+
+[[autodoc]] image_transforms.id_to_rgb
+
+[[autodoc]] image_transforms.normalize
+
+[[autodoc]] image_transforms.pad
+
+[[autodoc]] image_transforms.rgb_to_id
+
+[[autodoc]] image_transforms.rescale
+
+[[autodoc]] image_transforms.resize
+
+[[autodoc]] image_transforms.to_pil_image
+
+## ImageProcessingMixin
+
+[[autodoc]] image_processing_utils.ImageProcessingMixin