Docs: Streaming rendering issues with default nginx config

[llezhava]

I had issues with streaming rendering on the deployed application and turned out, that it was caused by the default nginx config.

I fixed it by adding proxy_buffering off; option.

Here is more info to it.

I think, that it will be good to be mentioned in the streaming rendering docs because nginx will be frequently used with remix, and I spent a couple of hours investigating this.