Subscription docs should mention CORS configuration

The docs aren't very clear on how to set up a subscription server.

e.g. `unable to upgrade *middleware.httpFancyWriter to websocket websocket: request origin not allowed by Upgrader.CheckOrigin:` is likely.

AddTransport after NewDefaultServer with a different transport.Websocket appears ineffective.

Look at the subscription server setup documentation in the gqlgen docs, likely under 'subscriptions' or 'server configuration'. Find where CORS configuration for WebSocket upgrades is mentioned or should be added. Check the code for the default server and transport.Websocket configuration to understand the correct placement. Verify by running a subscription example and ensuring the CORS error is resolved.

Written by the indexing model from the issue text.