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).

hardware/lib/README.md

@@ -0,0 +1,6 @@
+# Shared KiCad libraries
+
+Symbols (`*.kicad_sym`), footprints (`*.pretty/`) and 3D models (`3dmodels/`)
+used by more than one board. Add them to the board projects as project
+libraries with a path relative to the project (KiCad's KIPRJMOD variable), so
+the repository works on every computer.