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, 45 insertions, 0 deletions

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+# HID Generic In/Out
+
+A USB HID device that exchanges raw IN/OUT reports over a vendor-defined usage page, with no standard keyboard/mouse usage. It simply echoes back whatever the host sends.
+
+## What it does
+
+- Presents one HID interface built from the generic in/out report descriptor template (vendor usage page), with both an IN and an OUT endpoint.
+- Whenever the host sends data on the OUT endpoint (or via SET_REPORT), the device echoes the same bytes back to the host via `tud_hid_report`.
+- The LED blinks to indicate USB state (250 ms not mounted, 1000 ms mounted, 2500 ms suspended).
+- Because the reports carry no standard HID usage, the device is meant to be driven by a host-side tool rather than recognized as a keyboard/mouse. The example ships with `hid_test.js` (node-hid) and `hid_test.py` (Python `hid`) scripts to send and receive data.
+
+## USB Descriptors
+
+| Interface | Class driver |
+|-----------|--------------|
+| 0 | HID (generic in/out, raw vendor reports) |
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUD_HID            1
+#define CFG_TUD_HID_EP_BUFSIZE 64
+```
+
+## Building
+
+CMake:
+
+```bash
+mkdir build && cd build
+cmake -DBOARD=raspberry_pi_pico ..
+cmake --build .
+```
+
+Make:
+
+```bash
+make BOARD=raspberry_pi_pico all
+```
+
+## Try it
+
+After flashing, run the bundled host tool — `node hid_test.js` or `python3 hid_test.py` — to send a buffer to the device and observe the same data echoed back.