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, 26 insertions, 4 deletions

diff --git a/examples/host/msc_file_explorer/README.md b/examples/host/msc_file_explorer/README.md
index e220bedea..b80ca8f6c 100644
--- a/examples/host/msc_file_explorer/README.md
+++ b/examples/host/msc_file_explorer/README.md
@@ -27,16 +27,38 @@ CLI is presented over the board's serial console.
 | mv      | `mv <src> <dest>`  | Rename/move a file or directory                      |
 | rm      | `rm <file>`        | Remove a file                                        |
 
+## Configuration
+
+Notable `tusb_config.h` settings:
+
+```c
+#define CFG_TUH_ENABLED       1
+#define CFG_TUH_HUB           1
+#define CFG_TUH_MSC           1
+#define CFG_TUH_DEVICE_MAX    (3*CFG_TUH_HUB + 1)
+#define CFG_TUH_MSC_MAXLUN    4
+```
+
 ## Build
 
-Build for a specific board using CMake (see [Getting Started](https://docs.tinyusb.org/en/latest/getting_started.html)):
+CMake:
+
+```bash
+mkdir build && cd build
+cmake -DBOARD=raspberry_pi_pico ..
+cmake --build .
+```
+
+Make:
 
 ```bash
-# Example: build for Raspberry Pi Pico
-cmake -B build -DBOARD=raspberry_pi_pico -DFAMILY=rp2040 examples/host/msc_file_explorer
-cmake --build build
+make BOARD=raspberry_pi_pico all
 ```
 
+## FreeRTOS variant
+
+A FreeRTOS build is in `examples/host/msc_file_explorer_freertos` — identical MSC FatFS file-explorer CLI, running the host stack and CLI as FreeRTOS tasks.
+
 ## Usage
 
 1. Flash the firmware to your board.