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, 55 insertions, 0 deletions

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+# Audio Multi-Rate Microphone
+
+A single-channel USB microphone test example that supports multiple sample rates and adapts its descriptors to the negotiated USB bus speed — UAC1 at Full-Speed and UAC2 at High-Speed.
+
+## What it does
+
+- Enumerates as a single-channel microphone. The configuration descriptor returned depends on link speed: a UAC1 (Audio 1.0) configuration at Full-Speed and a UAC2 (Audio 2.0) configuration at High-Speed. A device-qualifier and other-speed-configuration descriptor are provided so it works at either speed.
+- Supports discrete sample rates of 32 kHz, 48 kHz and 96 kHz. At Full-Speed the rate is selected through the UAC1 endpoint sampling-frequency control; at High-Speed through the UAC2 clock-source frequency range.
+- Offers a 16-bit format; the High-Speed UAC2 configuration adds a second format with 24-bit samples carried in 32-bit slots, selected via the streaming interface's alternate setting.
+- An audio task generates an incrementing ramp signal sized to the current sample rate and sample width, and writes it to the IN endpoint every 1 ms. The ramp resets when the streaming interface is closed.
+- Handles both UAC1 and UAC2 control requests (mute, volume, sample frequency) dispatched on the active audio version.
+- Blinks the on-board LED to indicate USB state (not mounted / mounted / suspended).
+
+## USB Descriptors
+
+| Interface | Class driver |
+|-----------|--------------|
+| 0–1 | Audio (control + streaming), 1-channel microphone input — UAC1 at Full-Speed, UAC2 at High-Speed |
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUD_AUDIO                                        1
+#define CFG_TUD_AUDIO_FUNC_1_MAX_SAMPLE_RATE                96000
+#define CFG_TUD_AUDIO_FUNC_1_N_FORMATS                      2
+#define CFG_TUD_AUDIO_ENABLE_EP_IN                          1
+#define CFG_TUD_AUDIO_FUNC_1_N_CHANNELS_TX                  1
+#define CFG_TUD_AUDIO_FUNC_1_FORMAT_1_N_BYTES_PER_SAMPLE_TX 2   // 16-bit in 16-bit slots
+#define CFG_TUD_AUDIO_FUNC_1_FORMAT_1_RESOLUTION_RX         16
+#define CFG_TUD_AUDIO_FUNC_1_FORMAT_2_N_BYTES_PER_SAMPLE_TX 4   // 24-bit in 32-bit slots (UAC2 only)
+#define CFG_TUD_AUDIO_FUNC_1_FORMAT_2_RESOLUTION_RX         24
+#define CFG_TUD_AUDIO_FUNC_1_EP_IN_SZ_MAX                   TU_MAX(CFG_TUD_AUDIO10_FUNC_1_FORMAT_1_EP_SZ_IN, TU_MAX(CFG_TUD_AUDIO20_FUNC_1_FORMAT_1_EP_SZ_IN, CFG_TUD_AUDIO20_FUNC_1_FORMAT_2_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
+```
+
+## Try it
+
+The device appears as a single-channel USB microphone. On Linux, list it with `arecord -l` and record at a chosen rate with `arecord`, e.g. `arecord -D hw:CARD=MicNode -c 1 -f S16_LE -r 96000 test.wav`, trying 32000/48000/96000 to exercise rate switching. On a High-Speed host you can also select the 24-bit format (`-f S24_3LE` / `S32_LE` depending on the host). The included `src/plot_audio_samples.py` records and plots the signal.