Updates to computer vision section of the Preprocess doc (#21181)
* Extended the CV preprocessing section with more details and refactored the example
* added padding to the CV section, though it is a special case
* Added a tip about post processing methods
* make style
* link update
* Apply suggestions from review
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
* review feedback
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>