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/time_series_utils.md

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+<!--Copyright 2023 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
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+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
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+
+⚠️ Note that this file is in Markdown but contain specific syntax for our doc-builder (similar to MDX) that may not be
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+
+-->
+
+# Time Series Utilities
+
+This page lists all the utility functions and classes that can be used for Time Series based models.
+
+Most of those are only useful if you are studying the code of the time series models or you wish to add to the collection of distributional output classes.
+
+## Distributional Output
+
+[[autodoc]] time_series_utils.NormalOutput
+
+[[autodoc]] time_series_utils.StudentTOutput
+
+[[autodoc]] time_series_utils.NegativeBinomialOutput