docs: add proxy_buffering off to Nginx example for SSL proxy compatibility

[erseco]

This PR updates the documentation to include proxy_buffering off; in the Nginx configuration example for Mercure. This change improves compatibility when using Mercure behind an SSL-enabled Nginx proxy.

Problem

When Mercure is behind a Nginx reverse proxy with SSL, Nginx buffers responses by default. This can cause issues with long-lived SSE connections, leading to delayed message delivery and unexpected behavior for subscribers.

Solution

Disabling Nginx buffering with the following directive ensures that SSE messages are forwarded immediately to clients:

proxy_buffering off;

Changes

• Docs: Updated the Nginx configuration example to include proxy_buffering off;
• Notes: Added a brief explanation of why this directive is necessary when using SSL proxies.

Testing

• Tested with Mercure behind an Nginx SSL proxy.
• Observed that after adding proxy_buffering off;, SSE messages are delivered in real-time without delays.

Related issues

#854 #843 #857 #986

[dunglas]

Thanks!