Update docs: add custom.md for custom firmware guide and clarify ESP32-C3 pin usage

Signed-off-by: YoungSoo Shin <shinys000114@gmail.com>

docs/custom.md

@@ -0,0 +1,66 @@
+# Custom firmware
+
+If you want, you can create your own custom firmware.
+
+## Install Tools
+
+To build the source code, you need the following packages on Ubuntu 24.04.
+
+```bash
+sudo apt install nodejs npm nanopb
+```
+
+You need `esp-idf` version 5.4. It might be possible to build with the latest version, but it has not been confirmed.
+
+First, install the necessary tools for esp-idf.
+
+```bash
+sudo apt install git wget flex bison gperf python3 python3-pip python3-venv \
+  cmake ninja-build ccache libffi-dev libssl-dev dfu-util libusb-1.0-0 \
+
+```
+
+Next, clone the esp-idf tool.
+
+```bash
+mkdir -p ~/esp
+cd ~/esp
+git clone -b release/v5.4 --recursive https://github.com/espressif/esp-idf.git
+```
+
+You need to install tools such as a compiler and a debugger.
+
+```bash
+cd ~/esp/esp-idf
+./install.sh esp32c3
+```
+
+Please prepare to build the source by running the shell for using esp-idf.
+```bash
+. $HOME/esp/esp-idf/export.sh
+```
+## Build custom firmware based on PowerMate
+
+First, clone the PowerMate source code.
+```bash
+git clone https://github.com/shinys000114/odroid-powermate.git
+```
+
+After connecting the USB Type-C cable to the PowerMate and the host PC, run the command below to build and update PowerMate.
+```bash
+cd odroid-powermate
+idf.py app flash monitor
+```
+
+## Create a new project
+
+Create a project with the command below and create your own firmware.
+
+```bash
+idf.py create-project proj
+cd proj
+idf.py set-target esp32c3
+```
+
+For a more detailed guide, please refer to the link below.  
+[https://docs.espressif.com/projects/esp-idf/en/release-v5.4/esp32/get-started/linux-macos-setup.html#](https://docs.espressif.com/projects/esp-idf/en/release-v5.4/esp32/get-started/linux-macos-setup.html#)

docs/pin.md

@@ -13,6 +13,7 @@ minicom -D /dev/ttyACM0 -b 115200
 ## J2 UART Pin header
 
 Connected to the `U0RXD`, `U0TXD` system UART of the ESP32-C3. Currently not used on this board.
+It can be used as GPIO through appropriate configuration in esp-idf.
 
 ## J3 ODROID UART Connector
 
@@ -48,3 +49,4 @@ The output is set to 5.25V considering voltage drop. Please use with caution.
 **Reserved**
 
 SSD1309 OLED Connector
+It cannot be used as GPIO for other purposes because it shares the I2C bus with `PCA9557PW`, `INA3221`.