Preview feature is not working with forked repository

[hsbt]

@st0012 and I investigate that. The current cloudflare preview is not working with forked repository.

With my quick research, Netlify or similar services also disabled preview feature with forked repository. That caused by security reason. So, attacker can inject malicious code like javascript into preview pages.

We should prepare manually operation for building preview page from forked repository.

I remembered ruby.wasm triggered to build preview page with GitHub label.

/cc @kateinoigakukun

• https://github.com/ruby/ruby/pulls?q=is%3Apr+label%3APlayground+
• Namespace on read ruby#13226 (comment)

If rdoc and ruby/ruby prepared similar solution, It's good to new contributor and safe way to deploy documentation page.