chore(markdown): Updated plugin guide for v3

markdown/dev/guides/plugins/structure/en.md

@@ -0,0 +1,25 @@
+---
+title: Plugin structure
+order: 100
+---
+
+A FreeSewing plugin is a plain object with the following structure:
+
+```mjs
+Object plugin = {
+  String name,
+  String version,
+  Object hooks,
+  Object macros,
+  Array store,
+}
+```
+
+A plugin **must** have the `name` and `version` properties.
+The other properties are optional, and they map to the three different functionalities macros can provide:
+
+- `hooks`: Holds an object with lifecycle hooks the plugin wants to hook into
+- `macros`: Holds and object with macros the plugin provides
+- `store`: Holds and Array with store methods the plugin provides.
+
+Click on the links above for more details on the structure of these properties.