Redesign and clean up all icons across Zed (#35856)

- [x] Clean up unused and old icons
- [x] Swap SVG for all in-use icons with the redesigned version
- [x] Document guidelines

Release Notes:

- N/A

crates/ui/src/components/callout.rs

@@ -207,7 +207,7 @@ impl Component for Callout {
                 "Error with Multiple Actions",
                 Callout::new()
                     .icon(
-                        Icon::new(IconName::X)
+                        Icon::new(IconName::Close)
                             .color(Color::Error)
                             .size(IconSize::Small),
                     )