The buttons that are mapped to hotkeys (with the key mapping menu in options) will display the keymap on the button once you place an action on it.
By default, bars 1, 2, 3, and 4 all are mapped... to no modifier (ie. 1 thru 0 and the - and = key), and with modifier (alt+, ctrl+, and shft+). On the hotbars they display as either just 1 thru = or C+1 thru C+=, S+1, A+1, etc.
So, by default, the hotbars do show the shortcut keymap as long as there is an action bound to that slot.
Here's an example shot from "Anduril Reforged." It shows the unmodified hotbar, shift and control, as well as a couple slots from an unmapped bar and some modified keymaps (such as the S+E, Q, and E on bar 1).
If you want to use the 9-key (numpad) for your skills, try the key mapping settings in options. Set the hotbar slots you want to the numpad keys you want to control them. I'm not sure if the numpad is recognized differently than the normal keys, as I've never tried, but give it a whirl.