Merge pull request #3743 from hathach/claude/build-doc-skill

docs: add build-doc tooling and a README for every example

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+# CDC + MSC
+
+A composite USB device that exposes **two interfaces at once**:
+
+- **CDC** — a virtual serial port (`/dev/ttyACMx`, or a COM port on Windows) that echoes back everything it receives.
+- **MSC** — a small mass-storage disk that mounts as a removable drive.
+
+This is the canonical TinyUSB example for a multi-interface (composite) device.
+
+## What it does
+
+- **CDC:** reads incoming data and echoes it straight back. Pressing the on-board button sends a serial-state (UART) notification to the host.
+- **MSC:** presents an 8 KB FAT12 RAM disk (16 × 512-byte blocks — the smallest size Windows will mount) labelled `TinyUSB MSC`, containing a single `README.TXT`. The disk lives in RAM, so changes are not persistent.
+- **LED** shows bus state: 250 ms blink = not mounted, 1000 ms = mounted, 2500 ms = suspended.
+
+## USB Descriptors
+
+| Interface | Class driver |
+|-----------|--------------|
+| 0–1 | CDC (virtual serial) |
+| 2 | MSC (mass storage) |
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUD_CDC              1
+#define CFG_TUD_MSC              1
+#define CFG_TUD_CDC_RX_BUFSIZE   (TUD_OPT_HIGH_SPEED ? 512 : 64)
+#define CFG_TUD_CDC_TX_BUFSIZE   (TUD_OPT_HIGH_SPEED ? 512 : 64)
+#define CFG_TUD_MSC_EP_BUFSIZE   512
+```
+
+## Building
+
+CMake:
+
+```bash
+mkdir build && cd build
+cmake -DBOARD=raspberry_pi_pico ..
+cmake --build .
+```
+
+Make:
+
+```bash
+make BOARD=raspberry_pi_pico all
+```
+
+## FreeRTOS variant
+
+A FreeRTOS build is in `examples/device/cdc_msc_freertos` — identical CDC + MSC behavior, with the device, CDC, and LED-blink work split across FreeRTOS tasks.
+
+## Try it
+
+- **Serial:** open the CDC port (`screen /dev/ttyACM0`, PuTTY, …) and type — characters are echoed back.
+- **Storage:** a small `TinyUSB MSC` drive appears; open `README.TXT` to confirm it mounted.