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, 49 insertions, 0 deletions

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+# MIDI Test
+
+A USB MIDI device that continuously plays a fixed melody, useful for verifying USB MIDI enumeration and streaming.
+
+## What it does
+
+- Sends a repeating sequence of note-on / note-off messages (cable 0, channel 1) about every 286 ms.
+- Drains any incoming MIDI packets (read and discarded) so the host sender never blocks on a full input port.
+- Blinks the board LED to indicate USB state (not mounted / mounted / suspended).
+
+## USB Descriptors
+
+| Interface | Class driver |
+|-----------|--------------|
+| 0–1 | MIDI (Audio Control + MIDI Streaming) |
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUD_MIDI              1
+#define CFG_TUD_MIDI_RX_BUFSIZE   (TUD_OPT_HIGH_SPEED ? 512 : 64)
+#define CFG_TUD_MIDI_TX_BUFSIZE   (TUD_OPT_HIGH_SPEED ? 512 : 64)
+```
+
+## 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/midi_test_freertos` — identical MIDI melody playback, with the device, MIDI, and LED-blink work split across FreeRTOS tasks.
+
+## Try it
+
+The device shows up as a MIDI port. On Linux, list ports with `aseqdump -l`, then watch the incoming stream with `aseqdump -p <client:port>` (you should see a steady run of note on/off events), or route the port into a synth such as FluidSynth/qsynth to hear the melody.