Add Euro Office documentation and HTTP/3 (QUIC) Nginx configuration

[zerox80]

INFO:

opencloud-eu/opencloud-compose#271 has to be merged first.

Add Euro Office documentation and HTTP/3 (QUIC) Nginx configuration

Summary

• Add Euro Office configuration section to the admin documentation
• Extend the external proxy guide with Euro Office Nginx server blocks
• Add optional HTTP/3 (QUIC) Nginx configuration for advanced deployments

Details

New documentation section: Euro Office

Added docs/admin/configuration/euro-office/ with a comprehensive setup guide covering:

• Docker Compose configuration for Traefik and external proxy modes
• Running both Collabora and Euro Office simultaneously
• Environment variable reference table
• Exposed port mappings for external proxy setups
• DNS configuration for custom and .test domains

Extended external proxy guide

Updated docs/admin/getting-started/container/docker-compose/docker-external-proxy.md:

• Added Euro Office domains to DNS requirements, certbot commands, and temporary Nginx config
• Added .env examples for all three modes: Collabora only, Euro Office only, and both combined
• Added Nginx server blocks for Euro Office Document Server (port 9900) and its WOPI server (port 9302)
• Added a tip box about office suite choice linking to both configuration sections

New section: HTTP/3 (QUIC) support

Added an optional advanced section at the end of the external proxy guide with:

• Full Nginx configuration using listen 443 quic directives for all services (OpenCloud, Collabora, WOPI, Euro Office)
• upstream blocks with keepalive and map directive for cleaner WebSocket handling
• Comparison table highlighting key differences between HTTP/2 and HTTP/3 setups
• Notes on reuseport placement, Alt-Svc header, firewall requirements (UDP 443), and nginx version compatibility
• Notes on security benefits: QUIC is highly recommended from a security perspective. Deep Packet Inspection (DPI) by firewalls is mitigated as Server Name Indication (SNI) and other metadata are completely encrypted.
• Notes on network benefits: QUIC enforces strict DF (Don't Fragment) due to its built-in Datagram Packetization Layer Path MTU Discovery (DPLPMTUD) mechanism.

New files

File	Description
docs/admin/configuration/euro-office/_category_.json	Sidebar entry (position 31, after Collabora)
docs/admin/configuration/euro-office/index.md	Euro Office configuration guide

Modified files

File	Description
docs/admin/getting-started/container/docker-compose/docker-external-proxy.md	Euro Office Nginx blocks + HTTP/3 section

Test plan

• Verify Docusaurus builds without errors (pnpm build)
• Verify Euro Office section appears in sidebar under Configuration → Euro Office
• Verify all internal links resolve (Euro Office ↔ external proxy guide)
• Review Nginx config blocks for correctness
• Verify markdown linting passes (pnpm lint:md:fix)