docs(sdk): éclate et étend la documentation SDK par sujet

Remplace le README monolithique par un point d'entrée vers des pages dédiées
(manifest, activation/contexte, menus, commandes/feedback, layouts React,
fenêtres, services, packaging/distribution) pour couvrir #142-#144 et
faciliter la navigation.

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

docs/menus.md

@@ -0,0 +1,56 @@
+# Menus
+
+Menus are declared in the manifest and implemented by registering command
+handlers during activation.
+
+```json
+{
+  "contributes": {
+    "menus": [
+      {
+        "id": "hello-plugin.menu",
+        "label": "Hello Plugin",
+        "topLevel": true,
+        "order": 100
+      }
+    ],
+    "menuItems": [
+      {
+        "id": "hello-plugin.open.item",
+        "targetMenuId": "hello-plugin.menu",
+        "label": "Open Dashboard",
+        "command": "hello-plugin.open",
+        "order": 10,
+        "when": "projectOpen"
+      }
+    ]
+  }
+}
+```
+
+Then register the command id:
+
+```ts
+export function activate(ctx: ActivateContext): void {
+  const disposable = ctx.commands?.registerCommand("hello-plugin.open", async () => {
+    await ctx.services?.windows.open({ layoutType: "hello-plugin.dashboard" });
+  });
+  if (disposable) ctx.subscriptions.push(disposable);
+}
+```
+
+## Rules
+
+- A command can only be registered if at least one manifest menu item declares
+  that exact `command` id.
+- Missing command handlers are a no-op when the user clicks the menu item.
+- A handler owns precondition checks and feedback. See
+  [Commands And Feedback](commands-and-feedback.md).
+- Menu item `when` expressions are evaluated by the host; unsupported or false
+  conditions hide/disable the item according to host policy.
+
+## Targets
+
+`targetMenuId` can refer to a plugin top-level menu id or a host menu id exposed
+by IdeA. Prefer a plugin top-level menu for plugin-specific workflows and host
+menus only when the action naturally belongs beside native actions.