docs: update sidebar navigation

Fix sidebar_position collision between polar-types and web-app pages.
New nav order: Intro (1) → Python API (2) → Polar Types (3) → Web App (4).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: aeronauty

docs-site/docs/web-app.mdx

@@ -1,5 +1,5 @@
 ---
-sidebar_position: 3
+sidebar_position: 4
 title: Web App
 ---