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/indicator.rs

@@ -164,7 +164,7 @@ impl Component for Indicator {
                             ),
                             single_example(
                                 "Error",
-                                Indicator::icon(Icon::new(IconName::X))
+                                Indicator::icon(Icon::new(IconName::Close))
                                     .color(Color::Error)
                                     .into_any_element(),
                             ),