docs(vercel): say where to register the URL and add a troubleshooting entry

[k-kj0]

Deploying to Vercel with Rivet Cloud, I couldn't find where to register the URL (the step says "in the dashboard") and hit No runner config with name 'default' with nothing in Troubleshooting. This adds the dashboard path and one troubleshooting entry.

Question, not changed in this PR: after registering I also saw actor.no_runner_config_configured once. It stopped after I enabled more datacenters, but a later test with only Northern Virginia on a new actor also worked, so I couldn't isolate the cause. Happy to add an entry if you can tell me what triggers it.

Separately, an older version of this guide documented deploying a Hono app (vercel.json framework setting, .ts imports). This page now mentions only Next.js. Happy to restore that in a follow-up if you want it.

I used AI help to draft the wording and checked each change against my own deployment.

[the-company-company]

🟠 1 medium-severity finding

_{Reviewed commit 206b0ed.}

[the-company-company]

🟠 Medium · Troubleshooting omits the datacenter configuration that triggers this error

This error also occurs when default exists but is not enabled in any datacenter—the error itself says no config is available in any datacenter, and the registration step above makes selecting one mandatory. In that state, this guidance tells the reader only to confirm that the provider is listed and then offers no fix. Have the reader edit default and verify/select at least one datacenter as well as checking that the provider exists.

[the-company-company]

✅ No issues found

_{Reviewed commit bedd375.}