Spike: Next.js / React Server Components usage guide

reactfire works with Next.js but the full SSR/RSC story needs exploration before we can write authoritative guidance. Known pieces:

**Solved (client-side patterns):**
- Context providers need `'use client'` for App Router
- Browser-only SDKs (Analytics, etc.) also need `ssr: false` via `next/dynamic` to prevent SSR initialization — see #530

**Needs exploration:**
- How to use Firestore with React Server Components, including serialization constraints (Firestore objects can't cross the RSC boundary as-is)
- Reference: https://firebase.blog/posts/2026/06/firestore-serialization-react
- General guidance for which reactfire hooks are RSC-compatible vs. client-only

Outcome: a "Using reactfire with Next.js / RSC" section in docs covering both the simple `'use client'` pattern and the Firestore SSR serialization approach.

Related: #530, #334

Review related issues #530 and #334, then read the linked Firestore serialization reference. Explore Firestore use with React Server Components and determine which reactfire hooks are RSC-compatible versus client-only. Done means adding a “Using reactfire with Next.js / RSC” documentation section covering the 'use client' pattern and Firestore SSR serialization approach.

索引モデルが issue の本文から書いたものです。