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]>

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+# USB Speaker with Feedback
+
+A stereo USB speaker (output only) that uses an explicit isochronous feedback endpoint to keep the host's sample rate in sync, adapting its descriptors to the negotiated bus speed — UAC1 at Full-Speed and UAC2 at High-Speed.
+
+## What it does
+
+- Enumerates as a stereo speaker (host → device), 16-bit. The configuration returned depends on link speed: a UAC1 configuration at Full-Speed and a UAC2 configuration at High-Speed, with a device-qualifier and other-speed-configuration descriptor so it works at either speed.
+- Declares an explicit asynchronous feedback endpoint and reports rate via the FIFO-count feedback method (`tud_audio_feedback_params_cb`), letting the host adjust how many samples it sends so the device buffer neither under- nor over-runs.
+- The audio task drains the speaker FIFO every 1 ms into a dummy buffer (the data is discarded), reading roughly one millisecond of audio per cycle with a small periodic adjustment for the 44.1 kHz and 88.2 kHz rates.
+- Supports 44.1 kHz and 48 kHz at Full-Speed, plus 88.2 kHz and 96 kHz at High-Speed (UAC2). Handles UAC1 and UAC2 control requests (mute, volume, sample frequency).
+- When `CFG_AUDIO_DEBUG` is enabled (default), adds a HID interface that reports debug telemetry — current sample rate, alternate setting, mute/volume, and FIFO size/count/average — every 1 ms.
+- Blinks the on-board LED to indicate USB state, with a fast pattern while streaming.
+
+## USB Descriptors
+
+| Interface | Class driver |
+|-----------|--------------|
+| 0–1 | UAC2 audio (control + streaming), stereo speaker output with feedback endpoint |
+| 2 | HID (vendor report) — audio debug telemetry, present only when `CFG_AUDIO_DEBUG` is enabled |
+
+(UAC1 at Full-Speed, UAC2 at High-Speed.)
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUD_AUDIO                                 1
+#define CFG_AUDIO_DEBUG                               1   // adds HID debug interface
+#define CFG_TUD_HID                                   1   // present when CFG_AUDIO_DEBUG
+#define CFG_TUD_AUDIO_ENABLE_EP_OUT                   1   // speaker
+#define CFG_TUD_AUDIO_ENABLE_FEEDBACK_EP             1
+#define CFG_TUD_AUDIO_FUNC_1_N_CHANNELS_RX            2   // stereo
+#define CFG_TUD_AUDIO_FUNC_1_RESOLUTION_RX            16  // 16-bit
+#define CFG_TUD_AUDIO_FUNC_1_MAX_SAMPLE_RATE_FS       48000
+#define CFG_TUD_AUDIO_FUNC_1_MAX_SAMPLE_RATE_HS       96000
+```
+
+## Building
+
+CMake:
+
+```bash
+mkdir build && cd build
+cmake -DBOARD=raspberry_pi_pico ..
+cmake --build .
+```
+
+Make:
+
+```bash
+make BOARD=raspberry_pi_pico all
+```
+
+## Try it
+
+The device appears as a stereo USB speaker; select it as the system output or play to it with `aplay`/`speaker-test` (the audio is consumed and discarded). The feedback endpoint keeps the host's send rate matched to the device. With `CFG_AUDIO_DEBUG` enabled, `src/audio_debug.py` reads the HID telemetry stream and plots the FIFO level and sample rate.