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"

docs/src/visual-customization.md

@@ -430,6 +430,7 @@ Project panel can be shown/hidden with {#action project_panel::ToggleFocus} ({#k
     "indent_size": 20,              // Pixels for each successive indent
     "auto_reveal_entries": true,    // Show file in panel when activating its buffer
     "auto_fold_dirs": true,         // Fold dirs with single subdir
+    "sticky_scroll": true,          // Stick parent directories at top of the project panel.
     "scrollbar": {                  // Project panel scrollbar settings
       "show": null                  // Show/hide: (auto, system, always, never)
     },