microcontrollers: add new boards that are part of 0.19 release

Signed-off-by: deadprogram <ron@hybridgroup.com>

Author: deadprogram

content/docs/reference/microcontrollers/feather-rp2040.md

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+---
+title: "Adafruit Feather RP2040"
+weight: 3
+---
+
+The [Adafruit Feather RP2040]() is a tiny development board based on the Raspberry Pi [RP2040]() microcontroller.
+
+## Interfaces
+
+| Interface | Hardware Supported | TinyGo Support |
+| --------- | ------------- | ----- |
+| GPIO      | YES | YES |
+| UART      | YES | YES |
+| SPI      | YES | YES |
+| I2C      | YES | YES |
+| ADC      | YES | YES |
+| PWM      | YES | YES |
+
+## Machine Package Docs
+
+[Documentation for the machine package for the Adafruit Feather RP2040](../machine/feather-rp2040)
+
+## Flashing
+
+### UF2
+
+The Feather RP2040 comes with the [UF2 bootloader](https://github.com/Microsoft/uf2) already installed.
+
+### CLI Flashing
+
+- Plug your Feather RP2040 into your computer's USB port while holding down the RESET button on the board.
+- One plugged in, release the RESET button.
+- Flash your TinyGo program to the board using this command:
+
+    ```shell
+    tinygo flash -target=feather-rp2040 [PATH TO YOUR PROGRAM]
+    ```
+
+- The Feather RP2040 board should restart and then begin running your program.
+
+### Troubleshooting
+
+Any troubleshooting tips go here.
+
+## Notes
+
+You cannot yet use the USB port to the Feather RP2040 as a serial port. Instead `UART0` refers to the TX/RX pins on the board itself.

content/docs/reference/microcontrollers/nano-33-ble.md

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+---
+title: "Arduino Nano33 BLE"
+weight: 3
+---
+
+The [Arduino Nano33 BLE](h) is a very small ARM development board based on the Nordic Semiconductor [nrf52840](https://www.nordicsemi.com/eng/Products/nRF52840) processor.
+
+## Interfaces
+
+| Interface | Hardware Supported | TinyGo Support |
+| --------- | ------------- | ----- |
+| GPIO      | YES | YES |
+| UART      | YES | YES |
+| SPI      | YES | YES |
+| I2C      | YES | YES |
+| ADC      | YES | YES |
+| PWM      | YES | YES |
+
+## Machine Package Docs
+
+[Documentation for the machine package for the Arduino Nano33 IoT](../machine/arduino-nano33-ble)
+
+## Installing BOSSA
+
+In order to flash your TinyGo programs onto the Arduino Nano33 BLE board, you will need to install the "arduino_bossac" command line utility which is a special build of the [BOSSA command line utilities](https://github.com/shumatech/BOSSA).
+
+### macOS
+
+Instructions needed here.
+
+### Linux
+
+Instructions needed here.
+
+### Windows
+
+Instructions needed here.
+
+## Flashing
+
+Once you have installed the needed BOSSA command line utility, as in the previous section, you are ready to build and flash code to your Arduino Nano33 BLE board.
+
+### CLI Flashing
+
+- Plug your Arduino Nano33 BLE board into your computer's USB port.
+- Build and flash your TinyGo code using the `tinygo flash` command. This command flashes the Arduino Nano33 BLE with the blinky1 example:
+
+    ```shell
+    tinygo flash -target=arduino-nano33-ble examples/blinky1
+    ```
+
+- The Arduino Nano33 BLE board should restart and then begin running your program.
+
+### Troubleshooting
+
+Instructions needed here.
+
+## Notes
+
+You can use the USB port to the Arduino Nano33 BLE as a serial port. `UART0` refers to this connection.

content/docs/reference/microcontrollers/nano-rp2040.md

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+---
+title: "Nano RP2040"
+weight: 3
+---
+
+The [Nano RP2040]() is a tiny development board based on the Raspberry Pi [RP2040]() microcontroller.
+
+## Interfaces
+
+| Interface | Hardware Supported | TinyGo Support |
+| --------- | ------------- | ----- |
+| GPIO      | YES | YES |
+| UART      | YES | YES |
+| SPI      | YES | YES |
+| I2C      | YES | YES |
+| ADC      | YES | YES |
+| PWM      | YES | YES |
+
+## Machine Package Docs
+
+[Documentation for the machine package for the Nano RP2040](../machine/nano-rp2040)
+
+## Flashing
+
+### UF2
+
+The Nano RP2040 comes with the [UF2 bootloader](https://github.com/Microsoft/uf2) already installed.
+
+### CLI Flashing
+
+- Plug your Nano RP2040 into your computer's USB port while holding down the RESET button on the board.
+- One plugged in, release the RESET button.
+- Flash your TinyGo program to the board using this command:
+
+    ```shell
+    tinygo flash -target=nano-rp2040 [PATH TO YOUR PROGRAM]
+    ```
+
+- The Nano RP2040 board should restart and then begin running your program.
+
+### Troubleshooting
+
+Any troubleshooting tips go here.
+
+## Notes
+
+You cannot yet use the USB port to the Nano RP2040 as a serial port. Instead `UART0` refers to the TX/RX pins on the board itself.