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/extensions_ui/src/components/feature_upsell.rs

@@ -58,7 +58,7 @@ impl RenderOnce for FeatureUpsell {
                     el.child(
                         Button::new("open_docs", "View Documentation")
                             .icon(IconName::ArrowUpRight)
-                            .icon_size(IconSize::XSmall)
+                            .icon_size(IconSize::Small)
                             .icon_position(IconPosition::End)
                             .on_click({
                                 let docs_url = docs_url.clone();