chore(docs): use vercel ci again because GH secret problem on forks (#8142)

# Fallback to Vercel CI

Since we cannot share the vercel_token on forks we cannot deploy by
vercel CLI.
This PR reverts to the last working state by using vercel CI.

I will look into a fix with an npm script or a turbo config to ignore
builds on folder changes.

docs/vercel.json

@@ -1,6 +1,6 @@
 {
     "github": {
-        "enabled": false
+        "enabled": true
       },
     "cleanUrls": true,
     "rewrites": [