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/notifications/src/status_toast.rs

@@ -205,7 +205,7 @@ impl Component for StatusToast {
 
         let pr_example =
             StatusToast::new("`zed/new-notification-system` created!", cx, |this, _cx| {
-                this.icon(ToastIcon::new(IconName::GitBranchSmall).color(Color::Muted))
+                this.icon(ToastIcon::new(IconName::GitBranchAlt).color(Color::Muted))
                     .action("Open Pull Request", |_, cx| {
                         cx.open_url("https://github.com/")
                     })