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/repl/src/notebook/notebook_ui.rs

@@ -321,7 +321,7 @@ impl NotebookEditor {
                             .child(
                                 Self::render_notebook_control(
                                     "run-all-cells",
-                                    IconName::PlayOutlined,
+                                    IconName::PlayFilled,
                                     window,
                                     cx,
                                 )