Where should I upload the icon or image to add the link to the home screen on a mobile or tablet? Right now, it displays a letter instead of the icon or image I have in CP > Appearance > Icons & Logos.

Thanks in advance

System->Settings->Web App is what you would be looking for

I see, Thank you.

Do you know where I can find the documentation for that section?

There isnt really any documentation for the section. Is there something specific you are having issues with there?