Docs: `FeatureExtractor` rendering

[FBruzzesi]

Description

feature-extractors doc page renders automatically:

• All functionalities (properly and as expected)
• FeatureExtractor class that serves to register the polars ts namespace

I am not sure if this was intended to be for internal use only, yet outputs of these methods are way less documented that those of the original functions.

Example

• FeatureExtractor.absolute_energy method

  

• absolute_energy function

  

[baggiponte]

Been on my wishlist for a bit, thanks for opening the issue. I haven't dug into the docs yet but it should definitely be fixed. I think we could simply make the FeatureExtractor private. What do you think @abstractqqq?