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

diff --git a/examples/device/webusb_serial/README.md b/examples/device/webusb_serial/README.md
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+# WebUSB Serial
+
+A WebUSB device that acts as a web serial bridge, reachable directly from a WebUSB-capable browser (e.g. Chrome). It also exposes a standard CDC virtual serial port, and bridges traffic between the two.
+
+## What it does
+
+- Exposes a Vendor (WebUSB) interface plus a CDC virtual serial port.
+- Echoes any data received on either the WebUSB or the CDC interface back to **both** of them.
+- Serves a WebUSB landing-page URL (`example.tinyusb.org/webusb-serial/index.html`) via a BOS descriptor, so a supporting browser can offer to open the page.
+- Provides a Microsoft OS 2.0 descriptor so Windows auto-binds the WinUSB driver to the WebUSB interface.
+- Treats a vendor control request as a connect/disconnect signal from the browser; lights the on-board LED solid while the web serial is connected.
+- Blinks the on-board LED to reflect bus state: 250 ms unmounted, 1000 ms mounted, 2500 ms suspended.
+
+## USB Descriptors
+
+| Interface | Class driver |
+|-----------|--------------|
+| 0–1 | CDC (virtual serial) |
+| 2 | Vendor (WebUSB) |
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUD_CDC               1
+#define CFG_TUD_VENDOR            1
+#define CFG_TUD_CDC_RX_BUFSIZE    (TUD_OPT_HIGH_SPEED ? 512 : 64)
+#define CFG_TUD_CDC_TX_BUFSIZE    (TUD_OPT_HIGH_SPEED ? 512 : 64)
+#define CFG_TUD_VENDOR_RX_BUFSIZE (TUD_OPT_HIGH_SPEED ? 512 : 64)
+#define CFG_TUD_VENDOR_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
+```
+
+## Try it
+
+After flashing, open the landing page (`https://example.tinyusb.org/webusb-serial/index.html`) in a WebUSB-capable browser such as Chrome, click **Connect**, and select the device — the on-board LED lights solid once connected. Characters typed in the web page are echoed back, and are also mirrored to the CDC serial port (e.g. `/dev/ttyACM0`) and vice versa.
+
+On Linux/macOS you may need to install the udev rules from `examples/device/99-tinyusb.rules` for the browser to access the device.