docs: add optional HTTP/3 (QUIC) configuration guide for Nginx

[zerox80]

Hey everyone,

I thought it would be a good idea to add an HTTP/3 section to the external proxy guide for users who want to get the best possible performance out of their setup.

Since HTTP/3 runs over QUIC (UDP), it brings some massive benefits for a file sync solution like OpenCloud. Things like 0-RTT handshakes, no head-of-line blocking during large uploads, and connection migration for mobile clients really make a difference in everyday use. It also prevents MITM attacks via Deep Packet Inspection since the transport layer is fully encrypted

Here is a quick breakdown of what I added:

• A new section at the end of the docker-external-proxy.md document.
• A short overview explaining why users might want to bother setting up HTTP/3.
• Ready-to-use Nginx server blocks for OpenCloud alone, OpenCloud + Collabora, and OpenCloud + Euro Office. I used upstream blocks and a map directive to keep the websocket handling clean.
• Added warnings about the required UDP firewall rules and a quick command so users can check if their installed Nginx version actually supports QUIC.

The HTTP/3 configurations are totally independent of the standard HTTP/2 ones, so it shouldn't confuse users who just want a basic setup.

Let me know if you want anything changed!