docs: sync recent core, znet-sink, and zboard updates

[kovawx]

Summary

Synchronize the public documentation with recent implementation changes across Zero Core, ZNet Sink, and Zboard.

Zero Core

• Add a focused guide for standard HTTP forward-proxy requests on HTTP/Mixed inbounds.
• Document QUIC hostname resolution and the separation between network endpoint and TLS SNI.
• Document URLTest process-wide bounded concurrency, probe coalescing, result ordering, and GUI/controller waiting semantics.
• Link the new guide from the Core home page, guide index, and sidebar.

ZNet Sink

• Document managed local Mixed inbounds for subscription configurations, including the 127.0.0.1:7890 default and preservation of complete custom inbounds.
• Document Windows system-proxy restoration and the tray proxy terminal environment.
• Document adaptive URLTest waits, configuration-scoped latency history, nested groups, deduplicated targets, and batch completion recovery.
• Refresh subscription, feature, troubleshooting, guide-index, and sidebar content.

Zboard

• Add protocol-service documentation for VLESS/VMess TCP, WebSocket, and gRPC configuration, managed subscriber credentials, minimum Zero versions, and publication boundaries.
• Add subscription and traffic documentation covering generated Mixed inbounds, Base64 delivery semantics, camouflage redirects, selector behavior, exact-byte accounting, and adaptive units.
• Add DNS and certificate documentation covering revision-protected edits, exact Cloudflare record deletion, HTTP-01 public preflight, and Certbot DNS plugin fallbacks.
• Expand the project overview, node publication workflow, troubleshooting, guide index, and sidebar.

Scope

• Base branch: develop
• Head branch: agent/sync-project-updates-20260804
• 16 documentation files changed, including 5 new focused guides.
• Internal repository release-governance notes are not duplicated into the public documentation site.

Validation

• Compared the branch with develop: branch is ahead only and not behind.
• Reviewed the documented behavior against the current implementations and recent commits in all three repositories.
• Verified that every new page is linked from its project index and VitePress sidebar.
• pnpm check:build passed in Documentation CI, including project-link rules, Markdown rendering, local links, navigation reachability, VitePress build, and generated artifact checks.