Suggestion to add shape info in preprocessing

In the section about [preprocessing](https://huggingface.co/learn/audio-course/chapter1/preprocessing#preprocessing-audio-data), it would be useful to add type/shape information of data produced after pre processing the data.

Specifically, https://github.com/huggingface/audio-transformers-course/blob/ac81306fb8822fa8c4e2a43748be8ba31d8bb043/chapters/en/chapter1/preprocessing.mdx#L186 here it be very useful to add as a comment what is the type/shape of `input_features`. Is it 3d array of floats like [time, freq, ampl] ?

Open chapters/en/chapter1/preprocessing.mdx around line 186 and review the preprocessing section and the `input_features` example. Check the surrounding course material or referenced preprocessing behavior to confirm the data type and shape, then document that information where requested. Done means the example clearly explains the produced `input_features` structure.

Written by the indexing model from the issue text.