So you can actually add this to your keymap-desktop.json
file:
{
"command": "toggleEditors",
"accelerator": "Ctrl+G"
}
However I tried a number of different shortcuts and whilst I was able to go from markdown -> richtext I wasn't able to get it to go back again using the same shortcut. Might be worth having a go and seeing if it is possible with a different keybinding.