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, 48 insertions, 0 deletions

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+# HID Multiple Interfaces
+
+A composite USB device with two separate HID interfaces — one keyboard and one mouse — each with its own report descriptor, endpoint, and interface string.
+
+## What it does
+
+- Presents two independent HID interfaces: a keyboard (interface 0, "Keyboard Interface") and a mouse (interface 1, "Mouse Interface").
+- Polls the board button every 10 ms:
+  - Keyboard: while the button is held, sends the `A` keycode; releasing sends an empty report.
+  - Mouse: while the button is held, moves diagonally (+5, +5).
+- If the device is suspended, pressing the button issues a USB remote wakeup.
+- The LED blinks to indicate USB state (250 ms not mounted, 1000 ms mounted, 2500 ms suspended).
+
+## USB Descriptors
+
+| Interface | Class driver |
+|-----------|--------------|
+| 0 | HID (keyboard) |
+| 1 | HID (mouse) |
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUD_HID            2 // keyboard + mouse
+#define CFG_TUD_HID_EP_BUFSIZE 8
+```
+
+## 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, the board enumerates as two distinct HID devices, a keyboard and a mouse. Press and hold the button: the host receives repeated `A` key presses and the pointer drifts toward the bottom-right.