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, 40 insertions, 0 deletions

diff --git a/examples/device/board_test/README.md b/examples/device/board_test/README.md
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+# Board Test
+
+A minimal bring-up test that exercises a board's basic I/O without using the USB stack. Both the device and host stacks are disabled (`CFG_TUD_ENABLED`/`CFG_TUH_ENABLED` are `0`), so this is the first thing to run when porting to new hardware.
+
+## What it does
+
+- Blinks the on-board LED. The interval changes with the button: 1000 ms when the button is not pressed, 250 ms while it is held.
+- Prints `Hello from TinyUSB` over `stdout`/UART on each blink.
+- Echoes any character received on UART back out.
+
+(No USB class interfaces are present — this example does not enumerate as a USB device.)
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUD_ENABLED   0   // device stack disabled
+#define CFG_TUH_ENABLED   0   // host stack disabled
+```
+
+## Building
+
+CMake:
+
+```bash
+mkdir build && cd build
+cmake -DBOARD=raspberry_pi_pico ..
+cmake --build .
+```
+
+Make:
+
+```bash
+make BOARD=raspberry_pi_pico all
+```
+
+## Try it
+
+Flash the firmware and watch the on-board LED blink. Open the board's UART (serial console) to see `Hello from TinyUSB` printed repeatedly, and type characters to see them echoed back. Press and hold the button to speed up the blink rate.