[MR] user/bsps/arm/stm32f4: Expand BSP documentation

I’ve opened MR !240 to expand the stm32f4 BSP documentation page:

The page covered the OpenOCD + Nucleo workflow but was missing
sections for the STM32F411 Black Pill. Added a board support table,
stlink tools workflow as an alternative to OpenOCD, known hardware
limitations (Nucleo boards have no user USB connector, Black Pill
has no USB-to-serial bridge on the USB-C port), memory map, measured
flash/RAM footprint, and a USB peripheral section referencing gsoc#38.

Tested on STM32F411 Black Pill with stlink 1.7.0, arm-rtems7-gcc
15.2.0, RTEMS 7.0.0.e96fd2398b63bc6975094c648a9981e10731de5c on
Ubuntu 22.04.

Happy to get feedback and make any corrections needed.

Thanks,
Moksh