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/agent_ui/src/context_picker/completion_provider.rs

@@ -371,7 +371,7 @@ impl ContextPickerCompletionProvider {
                                                 line_range.end.row + 1
                                             )
                                             .into(),
-                                            IconName::Context.path().into(),
+                                            IconName::Reader.path().into(),
                                             range,
                                             editor.downgrade(),
                                         );
@@ -539,10 +539,10 @@ impl ContextPickerCompletionProvider {
             label: CodeLabel::plain(url_to_fetch.to_string(), None),
             documentation: None,
             source: project::CompletionSource::Custom,
-            icon_path: Some(IconName::Globe.path().into()),
+            icon_path: Some(IconName::ToolWeb.path().into()),
             insert_text_mode: None,
             confirm: Some(confirm_completion_callback(
-                IconName::Globe.path().into(),
+                IconName::ToolWeb.path().into(),
                 url_to_fetch.clone(),
                 excerpt_id,
                 source_range.start,