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/windows.md

@@ -0,0 +1,46 @@
+# Windows
+
+Plugin windows are detached OS windows that host an existing
+`contributes.layouts` layout. There is no separate window contribution type.
+
+```ts
+await ctx.services?.windows.open({
+  layoutType: "hello-plugin.dashboard",
+  state: { openedFrom: "menu" }
+});
+```
+
+## Contract
+
+- `layoutType` must match a layout type declared by the calling plugin.
+- The host validates the plugin is runtime-active and the layout exists before
+  opening or focusing the window.
+- Reopening the same plugin/layout pair focuses the existing window.
+- `state` is JSON-serializable initial window state. The mounted component can
+  later call `setState(next)` to update its local host state.
+- The window follows IdeA's focused project, matching detached native panel
+  behavior. If no project is focused, the host shows a neutral shell until one
+  is focused.
+
+## Return Value
+
+```ts
+const win = await ctx.services.windows.open({ layoutType: "hello-plugin.dashboard" });
+
+win.label;
+win.alreadyOpen;
+win.surface.layoutType;
+```
+
+The returned `label` is a host-owned window identity. Treat it as opaque.
+
+## Typical Menu Flow
+
+```ts
+ctx.commands?.registerCommand("hello-plugin.open-dashboard", async () => {
+  return ctx.services?.windows.open({
+    layoutType: "hello-plugin.dashboard",
+    state: { source: "menu" }
+  });
+});
+```