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

Documentation tooling:
- Add the `build-doc` skill and `tools/build_doc.py` wrapper for local
  Sphinx builds (clean / -W / open).
- Enable Markdown (MyST) in conf.py and auto-collect
  examples/{device,host,dual}/*/README.md into a 3-level Examples nav
  (Examples > Device/Host/Dual > example), noting each page's source
  location and normalizing headings to a single H1.
- Remove the stale `.claude/commands/build-doc.md`; point the AGENTS.md
  Documentation section at the skill.

Example docs:
- Add a README.md for every device/host/dual example: what it does, USB
  interface table, notable tusb_config.h settings, generic CMake + Make
  build steps, and how to try it.
- Fold each *_freertos variant into its base README, noting the FreeRTOS
  source path and any RTOS-specific behavior.

Generated docs/examples/ output is git-ignored. Builds clean with
`sphinx-build -W`.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

1 files changed, 59 insertions, 0 deletions

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+# Host: CDC/MSC/HID
+
+A USB host example that enumerates CDC serial, Mass Storage, and HID devices and reports their activity over the debug UART.
+
+## What it does
+
+- CDC (`CFG_TUH_CDC`): bidirectionally bridges the debug console and the attached CDC serial device — bytes typed on the debug UART are written to the device, and bytes received from the device are echoed back to the UART. Also supports common USB-serial adapters (FTDI, CP210x, CH34x, PL2303). On mount it prints the interface info and line coding (set to 115200 8N1 on enumeration).
+- MSC (`CFG_TUH_MSC`): on mount, issues a SCSI Inquiry and prints the drive's vendor/product/revision strings and its capacity (block count, block size, total MB).
+- HID (`CFG_TUH_HID`): receives reports and prints keyboard keystrokes as ASCII, mouse button state and cursor movement, and any other (generic) report as raw hex.
+- Blinks the board LED once per second.
+
+## Requirements
+
+The board must support USB host mode (provide VBUS to the connected device); some boards need an external USB-A port / host adapter.
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUH_ENABLED             1
+#define CFG_TUH_HUB                 1
+#define CFG_TUH_CDC                 1
+#define CFG_TUH_CDC_FTDI            1
+#define CFG_TUH_CDC_CP210X          1
+#define CFG_TUH_CDC_CH34X           1
+#define CFG_TUH_CDC_PL2303          1
+#define CFG_TUH_HID                 (3*CFG_TUH_DEVICE_MAX)
+#define CFG_TUH_MSC                 1
+#define CFG_TUH_DEVICE_MAX          (3*CFG_TUH_HUB + 1)
+#define CFG_TUH_HID_EPIN_BUFSIZE    64
+#define CFG_TUH_HID_EPOUT_BUFSIZE   64
+#define CFG_TUH_CDC_LINE_CONTROL_ON_ENUM  (CDC_CONTROL_LINE_STATE_DTR | CDC_CONTROL_LINE_STATE_RTS)
+#define CFG_TUH_CDC_LINE_CODING_ON_ENUM   { 115200, CDC_LINE_CODING_STOP_BITS_1, CDC_LINE_CODING_PARITY_NONE, 8 }
+```
+
+## 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/host/cdc_msc_hid_freertos` — identical CDC/MSC/HID host behavior, running the host stack in a FreeRTOS task with a software-timer LED.
+
+## How to use
+
+Plug a USB keyboard, mouse, flash drive, or serial device into the board's USB host port (a USB hub also works, so several can be connected at once). Watch the debug UART: keystrokes and mouse movement appear as you use the input devices, an attached flash drive prints its inquiry/size info, and for a serial device you can type into the debug console to forward characters to it and see its output echoed back.