boards/nucleo-f031k6: improve board doc

boards/nucleo-f031k6/doc.md

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+@defgroup    boards_nucleo-f031k6 STM32 Nucleo-F031K6
+@ingroup     boards_common_nucleo32
+@brief       Support for the STM32 Nucleo-F031K6
+
+## Overview
+
+The Nucleo-F031K6 is a board from ST's Nucleo family supporting ARM Cortex-M0
+STM32F031K6 microcontroller with 4KiB of RAM and 32KiB of Flash.
+
+## Pinout
+
+@image html pinouts/nucleo-f031k6-and-more.svg "Pinout for the Nucleo-F031K6 (from ST User Manual, UM1956, https://www.st.com/resource/en/user_manual/um1956-stm32-nucleo32-boards-mb1180-stmicroelectronics.pdf, page 31)" width=25%
+
+### MCU
+
+| MCU        |    STM32F031K6      |
+|:---------- |:------------------- |
+| Family     | ARM Cortex-M0       |
+| Vendor     | ST Microelectronics |
+| RAM        | 4KiB                |
+| Flash      | 32KiB               |
+| Frequency  | up to 48MHz         |
+| FPU        | no                  |
+| Timers     | 8 (5x 16-bit, 1x 32, 1x Systick, 1x Watchdog) |
+| ADC        | 1x 12-bit (13 channels) |
+| UARTs      | 1 (one USART)       |
+| SPIs       | 1                   |
+| CANs       | 0                   |
+| RTC        | 1                   |
+| I2Cs       | 1                   |
+| Vcc        | 2.0V - 3.6V         |
+| Datasheet  | [Datasheet](https://www.st.com/resource/en/datasheet/stm32f031k6.pdf) |
+| Reference Manual | [Reference Manual](https://www.st.com/resource/en/reference_manual/rm0091-stm32f0x1stm32f0x2stm32f0x8-advanced-armbased-32bit-mcus-stmicroelectronics.pdf) |
+| Programming Manual | [Programming Manual](https://www.st.com/resource/en/programming_manual/pm0215-stm32f0-series-cortexm0-programming-manual-stmicroelectronics.pdf) |
+| Board Manual | [Board Manual](https://www.st.com/resource/en/user_manual/um1956-stm32-nucleo32-boards-mb1180-stmicroelectronics.pdf) |
+
+## Flashing the Board Using ST-LINK Removable Media
+
+On-board ST-LINK programmer provides via composite USB device removable media.
+Copying the HEX file causes reprogramming of the board. This task
+could be performed manually; however, the cpy2remed (copy to removable
+media) PROGRAMMER script does this automatically. To program board in
+this manner, use the command:
+```
+make BOARD=nucleo-f031k6 PROGRAMMER=cpy2remed flash
+```
+@note This PROGRAMMER was tested using ST-LINK firmware 2.37.26. Firmware updates
+      can be found on [this STM webpage](https://www.st.com/en/development-tools/stsw-link007.html).