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).

external/README.md

@@ -0,0 +1,5 @@
+# External code
+
+Third-party code as git submodules, never edited here, for example:
+
+    git submodule add https://github.com/STMicroelectronics/stm32-mw-usbpd-core external/stm32-mw-usbpd-core