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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+# Host: HID Controller
+
+A USB host example that reads a USB game controller / gamepad (a HID device) and prints its inputs over the debug UART.
+
+## What it does
+
+- Enumerates HID devices (`CFG_TUH_HID`) and, on mount, prints the device's VID/PID.
+- Decodes reports from explicitly supported controllers — Sony DualShock 4 and a few compatible PS4 pads (Hori FC4, Hori PS4 Mini, ASW GG xrd) — printing the joystick axes (x, y, z, rz), D-pad direction, and pressed buttons (Square/Cross/Circle/Triangle, L1/R1/L2/R2, Share/Option/L3/R3, PS, touchpad click). Output is only printed when the report changes meaningfully.
+- Sends a periodic rumble output report back to the DualShock 4, with the motor intensities driven by the L2/R2 analog triggers.
+- Blinks the board LED once per second.
+
+Note: events are only shown for the explicitly supported controllers above; other HID devices enumerate but their reports are not decoded.
+
+## 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                 0
+#define CFG_TUH_HID                 (3*CFG_TUH_DEVICE_MAX)
+#define CFG_TUH_DEVICE_MAX          (3*CFG_TUH_HUB + 1)
+#define CFG_TUH_HID_EP_BUFSIZE      64
+```
+
+## 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 supported USB game controller (e.g. a Sony DualShock 4) into the board's USB host port. Move the sticks and press buttons — the changes are printed on the debug UART, and squeezing the L2/R2 triggers makes the controller rumble.