Convert rst files (#14888)

* Convert all tutorials and guides

* Convert all remaining rst to mdx

* Track and fix bad links

docs/source/main_classes/keras_callbacks.mdx

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+<!--Copyright 2021 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.
+-->
+
+# Keras callbacks
+
+When training a Transformers model with Keras, there are some library-specific callbacks available to automate common
+tasks:
+
+## PushToHubCallback
+
+[[autodoc]] keras_callbacks.PushToHubCallback