docs: restructure documentation into MkDocs Material site

docs/reference/api/service.md

@@ -0,0 +1,12 @@
+# Service API
+
+Auto-generated reference for the [`Service`][unshackle.core.service.Service] base class that every
+service plugin subclasses. See [Creating a Service](../../dev/creating-a-service.md) for a guided
+walkthrough and [Service Architecture](../../dev/service-architecture.md) for the wider object model.
+
+::: unshackle.core.service
+    options:
+      show_root_heading: false
+      show_source: true
+      members_order: source
+      heading_level: 2