docs(plugin): rewrite architecture + template for the external-only model

OpenCADStudio ships no built-in plugins, so the spec and scaffold no
longer describe the in-tree inventory path, demo_plugin, or the fictional
Storm Sewer reference. The architecture doc now covers the external cdylib
model end to end (ocs_plugin_api contract, export_plugin!, building +
publishing per-platform releases, runtime loading, the marketplace +
curated registry, the approach-B ABI caveat). The template becomes a
standalone cdylib crate (Cargo.toml + src/lib.rs + release workflow)
instead of an in-tree module.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/plugin-template/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "my-plugin"
+version = "0.1.0"
+edition = "2021"
+description = "An Open CAD Studio add-on."
+license = "GPL-3.0-only"
+
+# A plugin is a C-ABI dynamic library the host loads at runtime.
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+# The stable contract, with the runtime `host` surface (HostApi / BuiltinPlugin).
+ocs_plugin_api = { git = "https://github.com/HakanSeven12/OpenCADStudio", features = ["host"] }
+
+# Match the host's acadrust so the loaded library is binary-compatible
+# (approach B assumes the same toolchain + dependency versions).
+[patch.crates-io]
+acadrust = { git = "https://github.com/HakanSeven12/acadrust", branch = "main" }