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, 85 insertions, 0 deletions

diff --git a/examples/device/mtp/README.md b/examples/device/mtp/README.md
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+# MTP
+
+A USB Media Transfer Protocol device backed by a small in-RAM filesystem that the host can browse, read from and write to.
+
+## What it does
+
+- Presents a single MTP storage preloaded with two read-only objects: `readme.txt` and `tinyusb.png`.
+- Supports core MTP operations: get device info, open/close session, get storage IDs and storage info, enumerate object handles, get object info, get object and get partial object, and a device property (device friendly name).
+- Lets the host upload one additional object (SendObjectInfo / SendObject) into a 4 KB RAM buffer and delete objects (unless built read-only).
+- Handles MTP class control requests: cancel, device reset and get device status.
+- LED blink rate indicates bus state (not mounted / mounted / suspended).
+
+## USB Descriptors
+
+| Interface | Class driver |
+|-----------|--------------|
+| 0 | MTP |
+
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUD_MTP                     1
+#define CFG_TUD_MTP_EP_BUFSIZE          512
+#define CFG_TUD_MTP_EP_CONTROL_BUFSIZE  16 // should be enough to hold data in MTP control request
+
+// MTP device info
+#define CFG_TUD_MTP_DEVICEINFO_EXTENSIONS   "microsoft.com: 1.0; "
+#define CFG_TUD_MTP_DEVICEINFO_SUPPORTED_OPERATIONS \
+   MTP_OP_GET_DEVICE_INFO, \
+   MTP_OP_OPEN_SESSION, \
+   MTP_OP_CLOSE_SESSION, \
+   MTP_OP_GET_STORAGE_IDS, \
+   MTP_OP_GET_STORAGE_INFO, \
+   MTP_OP_GET_OBJECT_HANDLES, \
+   MTP_OP_GET_OBJECT_INFO, \
+   MTP_OP_GET_OBJECT, \
+   MTP_OP_GET_PARTIAL_OBJECT, \
+   MTP_OP_DELETE_OBJECT, \
+   MTP_OP_SEND_OBJECT_INFO, \
+   MTP_OP_SEND_OBJECT, \
+   MTP_OP_RESET_DEVICE, \
+   MTP_OP_GET_DEVICE_PROP_DESC, \
+   MTP_OP_GET_DEVICE_PROP_VALUE, \
+   MTP_OP_SET_DEVICE_PROP_VALUE
+
+#define CFG_TUD_MTP_DEVICEINFO_SUPPORTED_EVENTS \
+    MTP_EVENT_OBJECT_ADDED
+
+#define CFG_TUD_MTP_DEVICEINFO_SUPPORTED_DEVICE_PROPERTIES  \
+    MTP_DEV_PROP_DEVICE_FRIENDLY_NAME
+
+#define CFG_TUD_MTP_DEVICEINFO_CAPTURE_FORMATS \
+    MTP_OBJ_FORMAT_UNDEFINED, \
+    MTP_OBJ_FORMAT_ASSOCIATION, \
+    MTP_OBJ_FORMAT_TEXT, \
+    MTP_OBJ_FORMAT_PNG
+
+#define CFG_TUD_MTP_DEVICEINFO_PLAYBACK_FORMATS \
+    MTP_OBJ_FORMAT_UNDEFINED, \
+    MTP_OBJ_FORMAT_ASSOCIATION, \
+    MTP_OBJ_FORMAT_TEXT, \
+    MTP_OBJ_FORMAT_PNG
+```
+
+## 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 enumerates as an MTP device and shows up in a file browser (e.g. the Files/Explorer app, or `mtp-detect` / `mtp-files` from libmtp). You should see `readme.txt` and `tinyusb.png`; you can copy a small file onto the device and delete files.