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) <noreply@anthropic.com>

AGENTS.md

@@ -127,10 +127,7 @@ cd test/unit-test && ceedling test:all        # or ceedling test:test_fifo
 
 ## Documentation
 
-```bash
-pip install -r docs/requirements.txt
-cd docs && sphinx-build -b html . _build        # ~2.5 s
-```
+Sphinx docs in `docs/` (reStructuredText `.rst` or Markdown `.md` via MyST). Use the `build-doc` skill (`.claude/skills/build-doc/SKILL.md`) to build/preview locally (`sphinx-build`) and to regenerate auto-generated files (`tools/gen_doc.py` + `tools/gen_presets.py`) after adding a board or dependency.
 
 ## Code Size Metrics