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/traits/component_preview.rs

@@ -30,20 +30,20 @@ pub trait ComponentPreview: IntoElement {
         ExampleLabelSide::default()
     }
 
-    fn examples(_cx: &WindowContext) -> Vec<ComponentExampleGroup<Self>>;
+    fn examples(_cx: &mut WindowContext) -> Vec<ComponentExampleGroup<Self>>;
 
     fn custom_example(_cx: &WindowContext) -> impl Into<Option<AnyElement>> {
         None::<AnyElement>
     }
 
-    fn component_previews(cx: &WindowContext) -> Vec<AnyElement> {
+    fn component_previews(cx: &mut WindowContext) -> Vec<AnyElement> {
         Self::examples(cx)
             .into_iter()
             .map(|example| Self::render_example_group(example))
             .collect()
     }
 
-    fn render_component_previews(cx: &WindowContext) -> AnyElement {
+    fn render_component_previews(cx: &mut WindowContext) -> AnyElement {
         let title = Self::title();
         let (source, title) = title
             .rsplit_once("::")