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, 47 insertions, 0 deletions

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+# MSC Dual LUN
+
+A USB Mass Storage device that presents two logical units (LUNs), each a small in-RAM FAT12 disk.
+
+## What it does
+
+- Exposes two LUNs, each a 8 KB FAT12 RAM disk containing a `README.TXT` file (`TinyUSB 0` and `TinyUSB 1` volumes).
+- Both disks are read/write by default, so host changes are written back to RAM (unless built read-only).
+- Pressing the board button marks LUN1 as "not ready" to simulate removable media being absent (e.g. SD card removed).
+- Reports a custom SCSI inquiry: vendor `TinyUSB`, product `Mass Storage`, revision `1.0`.
+- Runs under no-OS, FreeRTOS or ThreadX (USB device and LED blink as separate tasks under an RTOS).
+- LED blink rate indicates bus state (not mounted / mounted / suspended).
+
+## USB Descriptors
+
+| Interface | Class driver |
+|-----------|--------------|
+| 0 | MSC (mass storage) |
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUD_MSC               1
+#define CFG_TUD_MSC_EP_BUFSIZE    512
+```
+
+## 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 plugging in, two removable drives appear on the host, each containing a `README.TXT`. Open or modify the files to exercise reads and writes. Press the board button to make the second drive report "not ready" (medium not present).