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>

tools/build_doc.py

@@ -0,0 +1,50 @@
+#!/usr/bin/env python3
+"""Build the TinyUSB Sphinx documentation locally.
+
+Thin wrapper around `sphinx-build` so a manual doc build is one command.
+`conf.py` auto-collects example READMEs, so no extra steps are needed.
+
+    python3 tools/build_doc.py            # build docs/_build/
+    python3 tools/build_doc.py -c -W -o   # clean, fail on warnings, open result
+"""
+import argparse
+import shutil
+import subprocess
+import sys
+import webbrowser
+from pathlib import Path
+
+TOP = Path(__file__).parent.parent.resolve()
+DOCS = TOP / "docs"
+BUILD = DOCS / "_build"
+
+
+def main():
+    p = argparse.ArgumentParser(description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter)
+    p.add_argument("-c", "--clean", action="store_true", help="remove docs/_build first")
+    p.add_argument("-W", "--strict", action="store_true", help="treat warnings as errors")
+    p.add_argument("-o", "--open", action="store_true", help="open the built docs in a browser")
+    args = p.parse_args()
+
+    if args.clean and BUILD.exists():
+        shutil.rmtree(BUILD)
+
+    cmd = ["sphinx-build", "-b", "html"]
+    if args.strict:
+        cmd.append("-W")
+    cmd += [str(DOCS), str(BUILD)]
+
+    print("+", " ".join(cmd))
+    rc = subprocess.call(cmd)
+    if rc != 0:
+        return rc
+
+    index = BUILD / "index.html"
+    print(f"\nDocs built: {index}")
+    if args.open:
+        webbrowser.open(index.as_uri())
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())