chore(markdown): Updated plugin guide for v3

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

@@ -0,0 +1,56 @@
+---
+title: Store methods
+order: 130
+---
+
+FreeSewing plugins can provide store methods, which facilitate data handling
+within a pattern.
+
+## Signature
+
+To provide one or more store methods, your plugin should have a `macros` property that
+is an array where each member is itself an array with two members:
+
+- The first member holds the key to attach the method to (in dot notation)
+- The second member holds the method to attach
+
+```mjs
+const myPlugin = {
+  name: 'example',
+  version: '0.0.1',
+  store: [
+    [
+      'log.panic',
+      function(store, ...params) {
+        store.setIfUnset('logs.panic', new Array())
+        store.push(...params)
+      }
+    ]
+  }
+}
+```
+
+## Arguments
+
+All store methods receive at least two arguments:
+
+- `store`: The store object itself
+- `...params`: All additional plugins that were passed to the store method
+
+## Overwriting store methods
+
+You are allowed to overwrite existing store methods.
+As it happens, this is how you should implement a custom logging solution, but overwriting the logging methods under the store's `log` key,
+
+However, the following methods cannot be overwritten:
+
+- `extend`
+- `get`
+- `push`
+- `set`
+- `setIfUnset`
+- `unset`
+
+## Return value
+
+Store methods do not need to return anything. If they do, it will be ignored.