Create window-sync.mdx for Window Sync feature

Add initial content for Window Sync documentation.

Signed-off-by: mr. m <[email protected]>

Author: mr-cheffy

content/docs/user-manual/window-sync.mdx

@@ -0,0 +1,7 @@
+---
+title: Window Sync
+description: Your opened windows mirror with each other instantly
+---
+import KeyboardShortcut from '@/components/KeyboardShortcut';
+
+Window Sync (TODO)