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/title_bar/src/collab.rs

@@ -518,7 +518,7 @@ impl TitleBar {
                             .mx_neg_0p5()
                             .h_full()
                             .justify_center()
-                            .child(Icon::new(IconName::ChevronDownSmall).size(IconSize::XSmall)),
+                            .child(Icon::new(IconName::ChevronDown).size(IconSize::XSmall)),
                     )
                     .toggle_state(self.screen_share_popover_handle.is_deployed()),
             )

crates/title_bar/src/title_bar.rs

@@ -344,7 +344,7 @@ impl TitleBar {
                         .child(
                             IconWithIndicator::new(
                                 Icon::new(IconName::Server)
-                                    .size(IconSize::XSmall)
+                                    .size(IconSize::Small)
                                     .color(icon_color),
                                 Some(Indicator::dot().color(indicator_color)),
                             )