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).

mech/README.md

@@ -0,0 +1,4 @@
+# Mechanics
+
+FreeCAD sources (`*.FCStd`) and STEP exports of enclosures, holders and
+heatsinks. FreeCAD backups (`*.FCBak`, `*.FCStd1`) are not committed.