[Docs] The "Process" how-to guide lacks description of `select_columns` function

[unifyh]

Feature request

The how to process dataset guide currently does not mention the select_columns function. It would be nice to include it in the guide.

Motivation

This function is a commonly requested feature (see this forum thread and #5468 #5474). However, it has not been included in the guide since its implementation by PR #5480.

Mentioning it in the guide would help future users discover this added feature.

Your contribution

I could submit a PR to add a brief description of the function to said guide.

[stevhliu]

Great idea, feel free to open a PR! :)