docs: add server gateway SSE guide

[benpsnyder]

Summary

• Adds a Server Gateway and SSE Relay how-to guide.
• Shows a framework-neutral TypeScript gateway shape using generated SpacetimeDB bindings from a server process.
• Demonstrates subscription callbacks feeding an event bus, SSE endpoint replay with Last-Event-ID, browser EventSource, reducer calls through authorized server endpoints, shutdown handling, and CLI smoke testing.
• Adds concrete TanStack Start examples for server-only gateway runtime files, createServerFn snapshot/mutation wrappers, server routes for SSE, and a route loader plus browser EventSource component.
• Links the guide from the Developer Resources index.

Why

This is a focused documentation slice from #5004. The issue asks for a first-class server-side realtime gateway pattern where a full-stack app server owns auth and the SpacetimeDB SDK connection, then relays live updates to browsers over SSE.

The guide keeps the core gateway example module-agnostic while also showing how the pattern maps into TanStack Start: loaders for initial snapshots, server functions for same-origin mutations, server routes for raw SSE streams, and .server.ts files for SpacetimeDB connections and secrets.

Validation

• git diff --check
• pnpm --dir docs typecheck
• pnpm --dir docs build

The docs build passes. It still prints the existing docusaurus-plugin-llms-txt warning for /docs/ask-ai/ask-ai, which is unrelated to this change.

Refs #5004

[cloutiertyler]

Hi @benpsnyder, thanks for your contribution to the docs. We don't accept draft PRs in our repo to keep our PRs relatively neat and manageable, so I'm going to close this for now, at least until you have a complete PR for us to review.