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, 54 insertions, 0 deletions

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+# Audio Test Microphone
+
+A minimal USB Audio Class 2.0 (UAC2) microphone that streams a generated test signal, useful for verifying the audio device stack end to end.
+
+## What it does
+
+- Enumerates as a UAC2 microphone with 1 input channel at 48 kHz, 16-bit.
+- An audio task fills a buffer with a continuously incrementing 16-bit ramp counter and writes it to the IN endpoint every 1 ms, simulating data from an I2S source. The counter resets when the streaming interface is closed.
+- Handles UAC2 control requests: feature-unit mute/volume, clock-source sample-rate and clock-validity, and input-terminal connector.
+- Blinks the on-board LED to indicate USB state (not mounted / mounted / suspended).
+
+## USB Descriptors
+
+| Interface | Class driver |
+|-----------|--------------|
+| 0–1 | UAC2 audio (control + streaming), 1-channel microphone input |
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUD_AUDIO                                 1
+#define CFG_TUD_AUDIO_FUNC_1_SAMPLE_RATE             48000
+#define CFG_TUD_AUDIO_ENABLE_EP_IN                    1
+#define CFG_TUD_AUDIO_FUNC_1_N_CHANNELS_TX            1
+#define CFG_TUD_AUDIO_FUNC_1_N_BYTES_PER_SAMPLE_TX    2   // 16-bit
+#define CFG_TUD_AUDIO_EP_SZ_IN                        TUD_AUDIO_EP_SIZE(TUD_OPT_HIGH_SPEED, CFG_TUD_AUDIO_FUNC_1_SAMPLE_RATE, CFG_TUD_AUDIO_FUNC_1_N_BYTES_PER_SAMPLE_TX, CFG_TUD_AUDIO_FUNC_1_N_CHANNELS_TX)
+#define CFG_TUD_AUDIO_FUNC_1_EP_IN_SZ_MAX            CFG_TUD_AUDIO_EP_SZ_IN
+```
+
+## 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/audio_test_freertos` — identical single-channel UAC2 test microphone, with the device, audio, and LED-blink work split across FreeRTOS tasks.
+
+## Try it
+
+The device appears as a single-channel USB microphone. On Linux, list it with `arecord -l` and capture with `arecord` (for example `arecord -D hw:CARD=MicNode -c 1 -f S16_LE -r 48000 test.wav`); the recorded samples form a rising ramp. The included `src/plot_audio_samples.py` records and plots the signal (requires `sounddevice` and `matplotlib`).