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, 68 insertions, 0 deletions

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+# DFU
+
+A USB Device Firmware Upgrade (DFU mode) device with two firmware partitions, demonstrating download, upload and manifestation.
+
+## What it does
+
+- Exposes a DFU interface with two alternate settings, one per simulated partition: alt 0 `FLASH`, alt 1 `EEPROM`.
+- Download (host to device): prints each received byte to stdout and immediately reports flashing complete.
+- Upload (device to host): returns a fixed string per partition (`Hello world from TinyUSB DFU! - Partition 0/1`), single block only.
+- Reports per-partition poll timeouts during download: 1 ms for FLASH (alt 0), 100 ms for EEPROM (alt 1).
+- Logs manifestation, abort and detach events.
+- Provides a BOS / Microsoft OS 2.0 descriptor so Windows binds the WinUSB driver.
+- LED blink rate indicates bus state (not mounted / mounted / suspended).
+
+## USB Descriptors
+
+| Interface | Class driver |
+|-----------|--------------|
+| 0 | DFU (two alternate settings) |
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUD_DFU               1
+#define CFG_TUD_DFU_XFER_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
+
+List the device in DFU mode:
+
+```bash
+dfu-util -l
+```
+
+Download firmware to a partition (a text file works well for this demo):
+
+```bash
+dfu-util -d cafe -a 0 -D firmware.bin    # partition 0 (FLASH)
+dfu-util -d cafe -a 1 -D firmware.bin    # partition 1 (EEPROM)
+```
+
+Upload from a partition back to the host:
+
+```bash
+dfu-util -d cafe -a 0 -U readback.bin
+dfu-util -d cafe -a 1 -U readback.bin
+```
+
+Downloaded bytes are echoed on the device's stdout; uploads return the partition's fixed string.