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

assets/icons/circle_help.svg

@@ -1 +1,5 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-circle-help-icon lucide-circle-help"><circle cx="12" cy="12" r="10"/><path d="M9.09 9a3 3 0 0 1 5.83 1c0 2-3 3-3 3"/><path d="M12 17h.01"/></svg>
+<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M8 14C11.3137 14 14 11.3137 14 8C14 4.68629 11.3137 2 8 2C4.68629 2 2 4.68629 2 8C2 11.3137 4.68629 14 8 14Z" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M6.54492 6.5C6.66248 6.16582 6.8945 5.88404 7.19991 5.70455C7.50532 5.52506 7.86439 5.45945 8.21354 5.51934C8.56268 5.57922 8.87937 5.76075 9.1075 6.03175C9.33564 6.30276 9.4605 6.64576 9.45997 7C9.45997 8.00002 7.95994 8.50003 7.95994 8.50003" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M8 10.5H8.005" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>