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/table.rs

@@ -160,7 +160,7 @@ impl ComponentPreview for Table {
         ExampleLabelSide::Top
     }
 
-    fn examples(_: &WindowContext) -> Vec<ComponentExampleGroup<Self>> {
+    fn examples(_: &mut WindowContext) -> Vec<ComponentExampleGroup<Self>> {
         vec![
             example_group(vec![
                 single_example(