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, 44 insertions, 0 deletions

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+# Host: MIDI Receive
+
+A USB host example that receives MIDI from a connected USB-MIDI device and prints the incoming bytes over the debug UART.
+
+## What it does
+
+- Enumerates USB-MIDI devices (`CFG_TUH_MIDI`) and, on mount, prints the interface index, device address, and the number of RX/TX cables.
+- On each received MIDI packet, reads the stream and prints the cable number followed by the raw MIDI bytes as hex.
+- Blinks the board LED once per second.
+
+## Requirements
+
+The board must support USB host mode (provide VBUS to the connected device); some boards need an external USB-A port / host adapter.
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUH_ENABLED             1
+#define CFG_TUH_HUB                 1
+#define CFG_TUH_DEVICE_MAX          (3*CFG_TUH_HUB + 1)
+#define CFG_TUH_MIDI                CFG_TUH_DEVICE_MAX
+```
+
+## Building
+
+CMake:
+
+```bash
+mkdir build && cd build
+cmake -DBOARD=raspberry_pi_pico ..
+cmake --build .
+```
+
+Make:
+
+```bash
+make BOARD=raspberry_pi_pico all
+```
+
+## How to use
+
+Plug a USB-MIDI device (keyboard, controller, or interface) into the board's USB host port. On attach the debug UART prints the mount info; then playing notes or moving controls prints lines like `Cable 0 rx: 90 3C 7F` with the raw MIDI bytes.