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).

CHANGELOG.md

@@ -0,0 +1,7 @@
+# Changelog: $REPO_NAME
+
+One section per prototype stage, newest first; the heading is the tag.
+
+## v0.1 (planned)
+
+- First boards.