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/keybinding.rs

@@ -188,7 +188,7 @@ fn icon_for_key(keystroke: &Keystroke, platform_style: PlatformStyle) -> Option<
         "up" => Some(IconName::ArrowUp),
         "down" => Some(IconName::ArrowDown),
         "backspace" => Some(IconName::Backspace),
-        "delete" => Some(IconName::Delete),
+        "delete" => Some(IconName::Backspace),
         "return" => Some(IconName::Return),
         "enter" => Some(IconName::Return),
         "tab" => Some(IconName::Tab),