How can STM32 on Maple Mini (leafLabs) be programmed using Keil?
On different boards with the same progamming interface (ie swd) it does not matter what board you're using. You should find SWCLK, SWDIO, GND, 3V3 pinouts on the board. All these pins are present on the MCU:



After finding, you should connect a programmer (ie ST-Link v2) to these pins. You should also install a driver for the programmer. In Keil - press Alt+F7, select Debug, select debugger in dropdown list (ST-link debugger), press settings, select port (SW). That's all.