docs: Add v4 migration docs

Author: aklinker1

docs/.vitepress/config.ts

@@ -9,6 +9,10 @@ const ogImage =
 const helpSidebar = {
   text: "Help",
   items: [
+    {
+      text: "Migrate to V4",
+      link: "/guide/migration-v4",
+    },
     {
       text: "Migrate to V3",
       link: "/guide/migration-v3",

docs/guide/migration-v4.md

@@ -0,0 +1,8 @@
+---
+lang: en-US
+title: Upgrading to v4
+---
+
+# Upgrading to v4
+
+If you include content script CSS files in your `manifest.json`, double check your build output to make sure there are no build conflicts (multiple inputs resulting in a single output, overriding the file's contents).