Add Quick Notes example plugin demonstrating core plugin surfaces

Mirrors the Bookmarks example (PAP-3575) with a company-scoped note
library: database namespace, scoped API routes, page/sidebar/widget/
settings UI slots. All 7 tests pass.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/plugins/examples/plugin-quick-notes-example/migrations/001_notes.sql

@@ -0,0 +1,12 @@
+CREATE TABLE plugin_quick_notes_b34a9f8617.notes (
+  id uuid PRIMARY KEY,
+  company_id uuid NOT NULL,
+  title text NOT NULL,
+  body text NOT NULL DEFAULT '',
+  tags text[] NOT NULL DEFAULT '{}',
+  created_at timestamptz NOT NULL DEFAULT now(),
+  updated_at timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE INDEX notes_company_idx
+  ON plugin_quick_notes_b34a9f8617.notes (company_id, created_at DESC);