Add placeholder git panel (#21894)

Adds a simple git placeholder panel for us to iterate from. Also
includes a number of assets from the git prototyping branch that we will
use.

Note: This panel is staff flagged for now.

Release Notes:

- N/A

crates/ui/src/components/button/button.rs

@@ -445,7 +445,7 @@ impl ComponentPreview for Button {
         "A button allows users to take actions, and make choices, with a single tap."
     }
 
-    fn examples(_: &WindowContext) -> Vec<ComponentExampleGroup<Self>> {
+    fn examples(_: &mut WindowContext) -> Vec<ComponentExampleGroup<Self>> {
         vec![
             example_group_with_title(
                 "Styles",