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/theme_selector/src/icon_theme_selector.rs

@@ -318,7 +318,7 @@ impl PickerDelegate for IconThemeSelectorDelegate {
                     Button::new("docs", "View Icon Theme Docs")
                         .icon(IconName::ArrowUpRight)
                         .icon_position(IconPosition::End)
-                        .icon_size(IconSize::XSmall)
+                        .icon_size(IconSize::Small)
                         .icon_color(Color::Muted)
                         .on_click(|_event, _window, cx| {
                             cx.open_url("https://zed.dev/docs/icon-themes");

crates/theme_selector/src/theme_selector.rs

@@ -376,7 +376,7 @@ impl PickerDelegate for ThemeSelectorDelegate {
                     Button::new("docs", "View Theme Docs")
                         .icon(IconName::ArrowUpRight)
                         .icon_position(IconPosition::End)
-                        .icon_size(IconSize::XSmall)
+                        .icon_size(IconSize::Small)
                         .icon_color(Color::Muted)
                         .on_click(cx.listener(|_, _, _, cx| {
                             cx.open_url("https://zed.dev/docs/themes");