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, 47 insertions, 0 deletions

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+# Host: Bare API
+
+A USB host example that drives the TinyUSB host stack through its low-level API directly, without the higher-level class-driver application layer.
+
+## What it does
+
+- Enumerates any attached device and, on mount, fetches and prints the full device descriptor (VID/PID, USB version, class, max packet size, etc.) over the debug UART.
+- Reads and prints the manufacturer, product, and serial-number string descriptors (UTF-16 to UTF-8).
+- Fetches and walks the configuration descriptor with a small hand-written parser.
+- For any HID interface found, opens its interrupt IN endpoint with the raw endpoint API (`tuh_edpt_open` / `tuh_edpt_xfer`) and continuously prints the incoming HID reports as raw hex bytes.
+- 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_DEVICE_MAX          (3*CFG_TUH_HUB + 1)
+#define CFG_TUH_ENUMERATION_BUFSIZE 256
+#define CFG_TUH_API_EDPT_XFER       1
+```
+
+## Building
+
+CMake:
+
+```bash
+mkdir build && cd build
+cmake -DBOARD=raspberry_pi_pico ..
+cmake --build .
+```
+
+Make:
+
+```bash
+make BOARD=raspberry_pi_pico all
+```
+
+## How to use
+
+Plug any USB device (a HID keyboard/mouse works well) into the board's USB host port. On attach you should see `Device attached` followed by the full device/string/configuration descriptor dump on the debug UART. If the device exposes a HID interface, its interrupt-IN reports are printed as hex as they arrive (e.g. when you press a key or move the mouse).