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, 56 insertions, 0 deletions

diff --git a/examples/device/dfu_runtime/README.md b/examples/device/dfu_runtime/README.md
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+# DFU Runtime
+
+A minimal device that exposes a DFU runtime interface, advertising to the host that it can be switched into DFU (bootloader) mode.
+
+## What it does
+
+- Exposes a single DFU runtime interface alongside normal operation.
+- On a DFU_DETACH request, it does not actually reboot into a bootloader; instead it speeds up the LED blink as an indicator (this example is intentionally minimal).
+- Provides a BOS / Microsoft OS 2.0 descriptor so Windows binds the WinUSB driver.
+- LED blink rate indicates bus state (not mounted / mounted / suspended / DFU detach).
+
+## USB Descriptors
+
+| Interface | Class driver |
+|-----------|--------------|
+| 0 | DFU Runtime |
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUD_DFU_RUNTIME 1
+```
+
+## Building
+
+CMake:
+
+```bash
+mkdir build && cd build
+cmake -DBOARD=raspberry_pi_pico ..
+cmake --build .
+```
+
+Make:
+
+```bash
+make BOARD=raspberry_pi_pico all
+```
+
+## Try it
+
+List the device; it appears in Runtime mode:
+
+```bash
+dfu-util -l
+```
+
+Request a switch to DFU mode:
+
+```bash
+dfu-util -e
+```
+
+This sends a DETACH request. Since the example is minimal it does not enter a real bootloader; it instead changes the LED to a fast blink as confirmation.