fix(docs-v3): rename README.md to home.md so Vercel serves it

Vercel stopped serving root-level README.md from static deployments
(deployments from identical source served it on May 4 but 404 it since
May 19), which broke v3.zod.dev — docsify fetches the homepage markdown
client-side. Point docsify at home.md instead.

packages/docs-v3/index.html

@@ -283,6 +283,8 @@
 
     <script>
       window.$docsify = {
+        // Vercel no longer serves root-level README.md from static deployments.
+        homepage: "home.md",
         subMaxLevel: 1,
         maxLevel: 3,
         auto2top: true,