docs: Format CSS with Prettier (#15333)

This PR formats the CSS in the docs with Prettier.

The indentation of these CSS files kept changing based on who last
touched them, so I added settings to the Zed repo to try and keep the
formatting intact until we can enforce it in CI.

Release Notes:

- N/A

.zed/settings.json

@@ -26,6 +26,10 @@
       "tab_size": 2,
       "formatter": "prettier"
     },
+    "CSS": {
+      "tab_size": 2,
+      "formatter": "prettier"
+    },
     "Rust": {
       "tasks": {
         "variables": {