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, 53 insertions, 0 deletions

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+# USBTMC
+
+A USB Test & Measurement Class (USBTMC) instrument with USB488/SCPI support, behaving like a simple programmable instrument.
+
+## What it does
+
+- Implements a USBTMC interface with USB488 capabilities and advertises SCPI support.
+- Responds to `*IDN?` with an identification string (`TinyUSB,ModelNumber,SerialNumber,FirmwareVer123456`).
+- Buffers other written messages and echoes them back as the query response, after a configurable delay.
+- A `delay <ms>` command adjusts that simulated response delay (0–10000 ms).
+- Maintains an IEEE-488.2 status byte (MAV/SRQ), and handles trigger, clear, and bulk abort requests.
+- Supports the USBTMC indicator-pulse request, which briefly pulses the board LED.
+- Uses an interrupt endpoint in addition to the bulk IN/OUT endpoints.
+
+## USB Descriptors
+
+| Interface | Class driver |
+|-----------|--------------|
+| 0 | USBTMC |
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUD_USBTMC                1
+#define CFG_TUD_USBTMC_ENABLE_INT_EP  1
+#define CFG_TUD_USBTMC_ENABLE_488     1
+```
+
+## Building
+
+CMake:
+
+```bash
+mkdir build && cd build
+cmake -DBOARD=raspberry_pi_pico ..
+cmake --build .
+```
+
+Make:
+
+```bash
+make BOARD=raspberry_pi_pico all
+```
+
+## Try it
+
+The device appears as a USBTMC test-and-measurement instrument. Talk to it with a VISA/SCPI tool (for example PyVISA). The included `visaQuery.py` opens the instrument and sends `*IDN?`:
+
+```bash
+python visaQuery.py
+```