Improve documentation about GitHub Actions

I’m not sure where I should propose this, so I’m adding this here:

The guide about deploying to Vercel from GitHub actions

is missing important feature about custom domains. I got an error when following the documentation, found in on GitHub discussion.

There is should be a link to this article:

Note: if someone is taking the time to deploy from GitHub action, he/she probably have a custom domain by now.

Another issue with docs is this issue:

It requires adding --scope <your org> to the Vercel CLI.

This can be solved by updating documentation.