Product repository template

Layout, proplan.toml, README/CHANGELOG, placeholder product image, .gitignore, KiBot/release/docgen workflows and TEMPLATE.md. Source of truth: contrib/product-template in B4L/proplan (cargo test --test product_template checks it as Gitea 1.27 creates a repository from it).

firmware/README.md

@@ -0,0 +1,7 @@
+# Firmware
+
+MCU code, one folder per target (an STM32CubeIDE or CMake project each), for
+example `firmware/main-mcu/`. Drivers shared by the targets go to
+`firmware/lib/`; third-party stacks (HAL, USB-PD middleware) are submodules in
+`external/`. IDE build folders (`Debug/`, `Release/`, `build/`) are not
+committed.

firmware/lib/README.md

@@ -0,0 +1,3 @@
+# Shared firmware code
+
+Drivers and modules used by more than one firmware target.