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

diff --git a/examples/device/video_capture_2ch/README.md b/examples/device/video_capture_2ch/README.md
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+# Dual-Channel Video Capture (UVC)
+
+A USB Video Class (UVC) device that exposes two independent video streaming functions on a single device.
+
+## What it does
+
+- Presents two UVC camera functions, both 128x96 at 10 fps.
+- Stream 0 sends YUY2 (uncompressed) color bars; stream 1 sends MJPEG color-bar frames.
+- Uses bulk streaming endpoints (`CFG_TUD_VIDEO_STREAMING_BULK`).
+- Adopts the frame interval requested by the host at stream commit, per stream.
+- Blinks the board LED to indicate USB state (not mounted / mounted / suspended).
+
+## USB Descriptors
+
+| Interface | Class driver |
+|-----------|--------------|
+| 0–1 | UVC video (control + streaming) — YUY2 |
+| 2–3 | UVC video (control + streaming) — MJPEG |
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUD_VIDEO                       2   // 2 video control interfaces
+#define CFG_TUD_VIDEO_STREAMING            2   // 2 video streaming interfaces
+#define CFG_TUD_VIDEO_STREAMING_EP_BUFSIZE 256
+#define CFG_TUD_VIDEO_STREAMING_BULK       1   // bulk streaming endpoints
+```
+
+## Building
+
+CMake:
+
+```bash
+mkdir build && cd build
+cmake -DBOARD=raspberry_pi_pico ..
+cmake --build .
+```
+
+Make:
+
+```bash
+make BOARD=raspberry_pi_pico all
+```
+
+## Try it
+
+Two webcam devices appear on the host. On Linux, list them with `v4l2-ctl --list-devices` and open each with `ffplay /dev/videoN` or a camera app — one shows YUY2 color bars, the other MJPEG color bars.