Add converter docs (#17593)

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+# Proton C
+
+The Proton C is an Arm STM32F303xC based drop-in replacement for the Pro Micro.
+
+<img src="https://i.imgur.com/GdsN1Rdh.jpg" alt="Proton C" />
+
+#### Features
+
+* Through-hole mounted USB-C Port
+* 32-bit 72MHz Cortex-M4 processor (STM32F303CCT6)
+* I2C, SPI, PWM, DMA, DAC, USART, I2S
+* 23x 3.3V I/O Ports
+* 1x 5V output for WS2812 LED chains
+* 256kB flash
+* 40kB RAM
+* AST1109MLTRQ speaker footprint
+* Reset button
+
+## Warnings
+
+Some of the PCBs compatible with Pro Micro have VCC (3.3V) and RAW (5V) pins connected (shorted) on the pcb. Using the Proton C will short 5V power from USB and regulated 3.3V which is connected directly to the MCU. Shorting those pins may damage the MCU on the Proton C.
+
+So far, it appears that this is only an issue on the Gherkin PCBs, but other PCBs may be affected in this way.
+
+In this case, you may want to not hook up the RAW pin at all.
+
+## Manual Conversion
+
+To use the Proton C natively, without having to specify `CONVERT_TO=proton_c`, you need to change the `MCU` line in `rules.mk`:
+
+```
+MCU = STM32F303
+BOARD = QMK_PROTON_C
+```
+
+Remove these variables if they exist:
+
+* `BOOTLOADER`
+* `EXTRA_FLAGS`
+
+Finally convert all pin assignments in `config.h` to the stm32 equivalents.
+
+| Pro Micro Left | Proton C Left | | Proton C Right | Pro Micro Right |
+|-----------|----------|-|----------|-----------|
+| `D3` | `A9` | | 5v | RAW (5v) |
+| `D2` | `A10` | | GND | GND |
+| GND | GND | | FLASH | RESET |
+| GND | GND | | 3.3v | VCC <sup>1</sup> |
+| `D1` | `B7` | | `A2` | `F4` |
+| `D0` | `B6` | | `A1` | `F5` |
+| `D4` | `B5` | | `A0` | `F6` |
+| `C6` | `B4` | | `B8` | `F7` |
+| `D7` | `B3` | | `B13` | `B1` |
+| `E6` | `B2` | | `B14` | `B3` |
+| `B4` | `B1` | | `B15` | `B2` |
+| `B5` | `B0` | | `B9` | `B6` |
+| `B0` (RX LED) | `C13` <sup>2</sup> | | `C13` <sup>2</sup> | `D5` (TX LED) |
+
+You can also make use of several new pins on the extended portion of the Proton C:
+
+| Left | | Right | 
+|------|-|-------|
+| `A4`<sup>3</sup> | | `B10` |
+| `A5`<sup>4</sup> | | `B11` |
+| `A6` | | `B12` |
+| `A7` | | `A14`<sup>5</sup> (SWCLK) |
+| `A8` | | `A13`<sup>5</sup> (SWDIO) |
+| `A15` | | RESET<sup>6</sup> |
+
+Notes:
+
+1. On a Pro Micro VCC can be 3.3v or 5v.
+2. A Proton C only has one onboard LED, not two like a Pro Micro. The Pro Micro has an RX LED on `D5` and a TX LED on `B0`.
+3. `A4` is shared with the speaker.
+4. `A5` is shared with the speaker.
+5. `A13` and `A14` are used for hardware debugging (SWD). You can also use them for GPIO, but should use them last.
+6. Short RESET to 3.3v (pull high) to reboot the MCU. This does not enter bootloader mode like a Pro Micro, it only resets the MCU.