Document project_panel.sticky_scroll (#36721)

Hat tip to: @watercubz in
https://github.com/zed-industries/zed/issues/22869#issuecomment-3183850576

Release Notes:

- N/A

docs/src/configuring-zed.md

@@ -3234,6 +3234,7 @@ Run the `theme selector: toggle` action in the command palette to see a current
     "scrollbar": {
       "show": null
     },
+    "sticky_scroll": true,
     "show_diagnostics": "all",
     "indent_guides": {
       "show": "always"