Improve markdown formatting for better readability

[sepehr21ar]

This PR improves the formatting of markdown files across the repository.

• Standardized headings and bullet points.
• Fixed inconsistent spacing and line breaks.
• Improved readability for contributors and new readers.

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[github-actions]

Preview

Preview and run these notebook edits with Google Colab:

• site/en/tutorials/keras/classification.ipynb

Rendered notebook diffs available on ReviewNB.com.

Format and style

Use the TensorFlow docs notebook tools to format for consistent source diffs and lint for style:

$ python3 -m pip install -U --user git+https://github.com/tensorflow/docs 
$ python3 -m tensorflow_docs.tools.nbfmt notebook.ipynb 
$ python3 -m tensorflow_docs.tools.nblint --arg=repo:tensorflow/docs notebook.ipynb 

If commits are added to the pull request, synchronize your local branch: git pull origin master